pw_env_setup: Add ability to override bazel BUILD file for CIPD repos

Change-Id: I65279e62bb53a4975c4a8acbb65825f4aa6e25ee
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/165530
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Erik Gilling <konkers@google.com>

pw_env_setup/bazel/cipd_setup/cipd_rules.bzl

@@ -48,9 +48,20 @@ def cipd_client_repository():
 cipd_repository = repository_rule(
     _cipd_repository_impl,
     attrs = {
-        "_cipd_client": attr.label(default = "@cipd_client//:cipd"),
-        "path": attr.string(),
-        "tag": attr.string(),
+        "_cipd_client": attr.label(
+            default = "@cipd_client//:cipd",
+            doc = "Location of the CIPD client binary (internal).",
+        ),
+        "build_file": attr.label(
+            allow_single_file = True,
+            doc = "A BUILD file to use instead of generating one internally.",
+        ),
+        "path": attr.string(
+            doc = "Path within CIPD where this repository lives.",
+        ),
+        "tag": attr.string(
+            doc = "Tag specifying which version of the repository to fetch.",
+        ),
     },
     doc = """
 Downloads a singular CIPD dependency to the root of a remote repository.

pw_env_setup/bazel/cipd_setup/internal/cipd_internal.bzl

@@ -152,15 +152,18 @@ def cipd_repository_impl(rctx):
         rctx: Repository context.
     """
     cipd_repository_base(rctx)
-    rctx.file("BUILD", """
+    if rctx.attr.build_file:
+        rctx.file("BUILD", rctx.read(rctx.attr.build_file))
+    else:
+        rctx.file("BUILD", """
 exports_files(glob(["**"]))
 
 filegroup(
     name = "all",
     srcs = glob(["**"]),
     visibility = ["//visibility:public"],
 )
-""")
+  """)
 
 def _cipd_path_to_repository_name(path, platform):
     """ Converts a cipd path to a repository name