Unbreak cpp_unittest rules for OSS buck2 builds

Summary:
- Make gtest main stub it's own library to link in. This resolves issues with
  multiple definitions of main(), which folly sometimes triggers. If the test
  includes its own definition, it will be preferred, otherwise the linked
  version will be used
- Turn gmock from an alias of test to it's own third party library. For macos
  they are packaged together, but on other systems they are published
  separately, and have separate pkgconfigs
- Add implicit deps to gtest/gmock for cpp_unittest rules. Internally these
  dependencies are implicitly added, so this matches that behavior.

Reviewed By: bigfootjon

Differential Revision: D62543160

fbshipit-source-id: 3ed1ba5ec3b22928b11fefa86133acab13c16382

shim/shims.bzl

@@ -49,6 +49,12 @@ _HEADER_SUFFIXES = (
     "-defs.tcc",
 )
 
+CPP_UNITTEST_MAIN_DEP = "shim//third-party/googletest:cpp_unittest_main"
+CPP_UNITTEST_LIB_DEPS = [
+    "shim//third-party/googletest:gtest",
+    "shim//third-party/googletest:gmock",
+]
+
 def _get_headers_from_sources(srcs):
     """
     Return the headers likely associated with the given sources
@@ -198,14 +204,12 @@ def cpp_unittest(
         extract_helper_lib = None,
         compiler_specific_flags = None,
         default_strip_mode = None,
-        srcs = [],
         **kwargs):
     _unused = (supports_static_listing, allocator, owner, tags, emails, extract_helper_lib, compiler_specific_flags, default_strip_mode)  # @unused
-    srcs = srcs + ["shim//third-party/googletest:gtest_main.cpp"]
+    deps = deps + [CPP_UNITTEST_MAIN_DEP] + CPP_UNITTEST_LIB_DEPS
     prelude.cxx_test(
         deps = _maybe_select_map(deps + external_deps_to_targets(external_deps), _fix_deps),
         visibility = visibility,
-        srcs = srcs,
         **kwargs
     )
 

shim/third-party/googletest/BUCK

@@ -5,7 +5,6 @@
 # License, Version 2.0 found in the LICENSE-APACHE file in the root directory
 # of this source tree.
 
-load("@shim//build_defs:export_files.bzl", "export_file")
 load("@shim//third-party:third_party.bzl", "third_party_library")
 
 oncall("open_source")
@@ -16,12 +15,15 @@ third_party_library(
     pkgconfig_name = "gtest",
 )
 
-alias(
+third_party_library(
     name = "gmock",
-    actual = ":gtest",
-    visibility = ["PUBLIC"],
+    homebrew_package_name = "googletest",
+    pkgconfig_name = "gmock",
 )
 
-export_file(
-    name = "gtest_main.cpp",
+cxx_library(
+    name = "cpp_unittest_main",
+    srcs = ["gtest_main.cpp"],
+    visibility = ["PUBLIC"],
+    deps = [":gtest"],
 )