Switches all tools to host configuration.

Refs #50.

`exec` is generally preferred, however it is not easily supported by `ts_project()` because we need to explicitly list all the Bazel artifact roots in `rootDirs` (https://bazelbuild.github.io/rules_nodejs/TypeScript.html#ts_project). `exec` dynamically computes artifact roots based on the target's configuration, which can change arbitrarily based on build flags and how targets are used. It's not fit to be hard-coded in a `tsconfig.json`. I left a `TODO` to switch back to `exec` after updating to `rules_js`, since it should be able to fix this particular rough edge.

packages/rules_prerender/multi_inject_resources.bzl

@@ -50,7 +50,7 @@ multi_inject_resources = rule(
         "_injector": attr.label(
             default = "//tools/internal:resource_injector",
             executable = True,
-            cfg = "exec",
+            cfg = "host", # TODO(#48): Use `exec`.
         ),
     },
 )

packages/rules_prerender/prerender_pages_unbundled.bzl

@@ -232,7 +232,7 @@ _multi_extract_annotations = rule(
         "_extractor": attr.label(
             default = "//tools/internal:annotation_extractor",
             executable = True,
-            cfg = "exec",
+            cfg = "host", # TODO(#48): Use `exec`.
         ),
     },
     doc = """Extracts annotations from HTML files in the provided directory.""",

packages/rules_prerender/prerender_resources.bzl

@@ -148,7 +148,7 @@ _prerender_resources = rule(
         "renderer": attr.label(
             mandatory = True,
             executable = True,
-            cfg = "exec",
+            cfg = "host", # TODO(#48): Use `exec`.
         ),
     },
 )

packages/rules_prerender/web_resources.bzl

@@ -199,7 +199,7 @@ _web_resources_rule = rule(
         "_packager": attr.label(
             default = "//tools/internal:resource_packager",
             executable = True,
-            cfg = "exec",
+            cfg = "host", # TODO(#48): Use `exec`.
         ),
     },
 )