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
diff --git a/bp2build/Android.bp b/bp2build/Android.bp
index 99d706c..c74f902 100644
--- a/bp2build/Android.bp
+++ b/bp2build/Android.bp
@@ -11,6 +11,7 @@
         "build_conversion.go",
         "bzl_conversion.go",
         "configurability.go",
+        "constants.go",
         "conversion.go",
         "metrics.go",
     ],