Support three options for converting modules with unconverted
dependencies
1. (default) Warn when converting a module if it has unconverted deps.
2. Error when encountering a module with unconverted deps. (not hooked
up yet)
Test: build/bazel/ci/bp2build.sh
Test: build/bazel/ci/mixed_libc.sh
Test: BP2BUILD_ERROR_UNCONVERTED=1 build/bazel/ci/bp2build.sh with
unconverted deps -- get appropriate error
Bug: 181155349
Change-Id: Ifaabf0cd2e43e963366dc137159c705294165c3d