bp2build: automatically convert all genrules.

Results of `cquery //bionic/...`: https://paste.googleplex.com/5608885982330880?raw

Note that this CL does not contain the global module name mapping to
fully qualified label lookup mechanism, so while the BUILD targets are
queryable, they are not buildable (yet).

Test: GENERATE_BAZEL_FILES=true m nothing && ./build/bazel/scripts/bp2build-sync.sh write && bazel cquery //bionic/...

Change-Id: I6f2316222723d8b612d9b041ca82a188cd05bac9