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