feat(bazel): create toolchain alias rule for browser metadata targets

Creates a toolchain alais rule for exposing template variables extracted
from browser metadata (such as chromium or firefox)

These toolchain aliases can then be consumed in the `toolchains`
attribute of _all_ rules (as this is a common attribute supported
by all rules in Bazel). The toolchain aliases within the `toolchains`
attribute of rules, such as a `genrule` can then be used to access
certain named files through Make variable expansion within Bazel.

bazel/browsers/browser_configure.bzl

@@ -1,16 +1,21 @@
 load("@io_bazel_rules_webtesting//web/internal:metadata.bzl", "metadata")
 load("@io_bazel_rules_webtesting//web/internal:provider.bzl", "WebTestInfo")
 
-"""Converts the specified label to a manifest path"""
+NamedFilesInfo = provider(
+    doc = "Provider exposing the named files of an extracted browser archive.",
+    fields = {
+        "value": "Dictionary of keys and their corresponding manifest paths",
+    },
+)
 
 def _label_to_manifest_path(label):
+    """Converts the specified label to a manifest path"""
     if label.package != "":
         return "%s/%s" % (label.workspace_name, label.package)
     return label.workspace_name
 
-"""Implementation of the `browser_configure` rule."""
-
 def _browser_configure_impl(ctx):
+    """Implementation of the `browser_configure` rule."""
     named_files = {}
     base_dir = _label_to_manifest_path(ctx.label)
 
@@ -32,6 +37,7 @@ def _browser_configure_impl(ctx):
     return [
         DefaultInfo(runfiles = ctx.runfiles(files = ctx.files.files)),
         WebTestInfo(metadata = ctx.outputs.web_test_metadata),
+        NamedFilesInfo(value = ctx.attr.named_files),
     ]
 
 """

bazel/browsers/browser_toolchain_alias.bzl

@@ -0,0 +1,29 @@
+load("//bazel/browsers:browser_configure.bzl", "NamedFilesInfo")
+
+def _browser_toolchain_alias_impl(ctx):
+    base_path = ctx.attr.metadata.label.workspace_name
+    named_files = ctx.attr.metadata[NamedFilesInfo].value
+    template_variables = {}
+
+    for key, value in named_files.items():
+        template_variables[key] = "external/%s/%s" % (base_path, value)
+
+    return [
+        platform_common.TemplateVariableInfo(template_variables),
+    ]
+
+browser_toolchain_alias = rule(
+    doc = """
+      Exposes a toolchain alias exposing the template variables for all named files
+      included in the browser metadata. These variables can then be consumed within Bazel
+      make variable expansion.
+    """,
+    implementation = _browser_toolchain_alias_impl,
+    attrs = {
+        "metadata": attr.label(
+            doc = """Metadata target that provides the browser named files.""",
+            mandatory = True,
+            providers = [NamedFilesInfo, DefaultInfo],
+        ),
+    },
+)