Adds _prerender_for_test to a number of rules_prerender macros.

Fixes #10.

This is a public API which provides a reference to the `ts_library()` target which compiles the user's `srcs`. It allows users to easily invoke their prerender code for testing purposes. This is specifically marked `testonly` because it is focused on the single testing use case.

This target's existence technically breaks an invariant of `prerender_component()`, in that it only depends on a single piece of a complex component (it does not include client side scripts, CSS, or othe resources). As a result, I'm trying to restrict and discourage usage like this as much as possible. Unit testing is just the one exception where this really gets in the way, so hopefully this `testonly` target will satisfy that use case sufficiently without providing an abusable hook for users to exploit and workaround the guarantees provided by `prerender_component()`.

examples/site/blog/BUILD.bazel

@@ -23,7 +23,7 @@ ts_library(
     srcs = ["blog_test.ts"],
     testonly = True,
     deps = [
-        ":blog_page_component_prerender",
+        ":blog_prerender_for_test",
         "@npm//@types/jasmine",
         "@npm//node-html-parser",
     ],

examples/site/components/base/BUILD.bazel

@@ -27,7 +27,7 @@ ts_library(
     srcs = ["base_test.ts"],
     testonly = True,
     deps = [
-        ":base_prerender",
+        ":base_prerender_for_test",
         "@npm//@types/jasmine",
         "@npm//node-html-parser",
     ],

examples/site/components/counter/BUILD.bazel

@@ -15,7 +15,7 @@ ts_library(
     srcs = ["counter_prerender_test.ts"],
     testonly = True,
     deps = [
-        ":counter_prerender",
+        ":counter_prerender_for_test",
         "@npm//@types/jasmine",
         "@npm//node-html-parser",
     ],

examples/site/components/footer/BUILD.bazel

@@ -26,7 +26,7 @@ ts_library(
     srcs = ["footer_test.ts"],
     testonly = True,
     deps = [
-        ":footer_prerender",
+        ":footer_prerender_for_test",
         "@npm//@types/jasmine",
         "@npm//node-html-parser",
     ],

examples/site/components/header/BUILD.bazel

@@ -23,7 +23,7 @@ ts_library(
     srcs = ["header_test.ts"],
     testonly = True,
     deps = [
-        ":header_prerender",
+        ":header_prerender_for_test",
         "@npm//@types/jasmine",
         "@npm//node-html-parser",
     ],

packages/rules_prerender/prerender_component.bzl

@@ -45,10 +45,29 @@ def prerender_component(
         deps: `prerender_component()` dependencies for this component.
         testonly: See https://docs.bazel.build/versions/master/be/common-definitions.html.
         visibility: See https://docs.bazel.build/versions/master/be/common-definitions.html.
+    
+    Outputs:
+        `%{name}`: A compiled prerender component for use by other
+            `prerender_*()` rules/macros. This is not a real target as this
+            actually macro outputs several targets at `%{name}_*`. These are
+            **not** intended to be directly depended upon because it would be
+            very easy to accidentally depend on one part of a component (such as
+            the prerendered HTML) without also depending all the other parts
+            (such as the CSS) for it to work as expected. Instead of using the
+            outputs of this macro directly, use another `prerender_*()`
+            macro/rule to depend on and use this correctly.
+        `%{name}_prerender_for_test`: There is one exception which can be
+            directly depended upon. This is an alias to the `ts_library()`
+            target which compiles the `srcs` of this macro. It does **not**
+            include client side scripts, CSS, or other resources used by the
+            component. It is intended purely for testing purposes to assert that
+            prerender logic works as intended and is marked `testonly`
+            accordingly.
     """
 
+    prerender_lib = "%s_prerender" % name
     ts_library(
-        name = "%s_prerender" % name,
+        name = prerender_lib,
         srcs = srcs,
         tsconfig = tsconfig,
         data = data,
@@ -57,6 +76,12 @@ def prerender_component(
         visibility = visibility,
     )
 
+    native.alias(
+        name = "%s_prerender_for_test" % name,
+        actual = ":%s" % prerender_lib,
+        testonly = True,
+    )
+
     ts_library(
         name = "%s_scripts" % name,
         srcs = [],

packages/rules_prerender/prerender_pages.bzl

@@ -74,6 +74,9 @@ def prerender_pages(
             included. CSS is inlined directly in the HTML documents in a
             `<style />` tag. Non-HTML files are included as well, but not
             modified.
+        %{name}_prerender_for_test: An alias to the `ts_library()` target which
+            compiles the `src` of this macro marked as `testonly`. This provides
+            a simple hook for unit testing prerender logic.
     
     Args:
         name: The name of this rule.
@@ -113,6 +116,12 @@ def prerender_pages(
         visibility = visibility,
     )
 
+    native.alias(
+        name = "%s_prerender_for_test" % name,
+        actual = ":%s_prerender_for_test" % prerender_name,
+        testonly = True,
+    )
+
     bundle = "%s_bundle" % name
     if bundle_js:
         # Bundle all client-side scripts at `%{name}_bundle.js`.

packages/rules_prerender/prerender_pages_unbundled.bzl

@@ -65,6 +65,9 @@ def prerender_pages_unbundled(
             the page.
         %{name}_resources: A `web_resources()` target containing all the
             transitively used resources.
+        %{name}_prerender_for_test: An alias to the `ts_library()` target which
+            compiles the `src` of this macro marked as `testonly`. This provides
+            a simple hook for unit testing prerender logic.
     
     Args:
         name: The name of this rule.
@@ -99,10 +102,17 @@ def prerender_pages_unbundled(
         testonly = testonly,
     )
     component_prerender = "%s_prerender" % component
+    component_prerender_for_test = "%s_prerender_for_test" % component
     component_scripts = "%s_scripts" % component
     component_styles = "%s_styles" % component
     component_resources = "%s_resources" % component
 
+    native.alias(
+        name = "%s_prerender_for_test" % name,
+        actual = ":%s" % component_prerender_for_test,
+        testonly = True,
+    )
+
     # Get the generated JS file path for the user provided TypeScript source.
     prerender_js = "%s_prerender_js" % name
     native.filegroup(