Use handcrafted build targets in bp2build

If both bp2build_available and label are specified, label will be
preferred.

Initially, we copy the entire BUILD.bazel file. Eventually we may move
this to use bazel query for a more accurate result.

Test: go test *
Test: build/bazel/scripts/milestone-2/demo.sh full
Test: GENERATE_BAZEL_FILES=true m nothing
      edit bionic/libc/tools/BUILD.bazel
      GENERATE_BAZEL_FILES=true m nothing and verify changes picked up
Bug: 180516554
Change-Id: I43025583300e6b10d2c18032cd4a76237b578d59

bp2build/constants.go

@@ -0,0 +1,25 @@
+// Copyright 2020 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package bp2build
+
+var (
+	// When both a BUILD and BUILD.bazel file are exist in the same package, the BUILD.bazel file will
+	// be preferred for use within a Bazel build.
+
+	// The file name used for automatically generated files. Files with this name are ignored by git.
+	GeneratedBuildFileName = "BUILD"
+	// The file name used for hand-crafted build targets.
+	HandcraftedBuildFileName = "BUILD.bazel"
+)