bp2build: automatically convert all filegroups.

See build_conversion_test.go for expected outputs.

Test: build_conversion_test.go
Test: GENERATE_BAZEL_FILES=true m nothing &&
./build/bazel/scripts/bp2build-sync.sh write && bazel build
//bionic/libc/tools/...
Test: bazel query //... --config=queryview

Change-Id: I3c54b96c0812f1ea4ab2c95da1bff3d7c5cc4006