we can't use runfiles directory on Windows due to path lengths, so swap
to manifest strategy. Parsing the manifest is a bit complex and the
format is changing in Bazel upstream, so pull in the official Rust
library (via a small hack to make it importable...) and cleanup all the
associated logic to work cleanly in both bazel and cargo without extra
confusion
As noted in the comment, this was causing a problem for me locally
because Sapling backed up some files under `.git/sl` named `BUILD.bazel`
and so Bazel tried to parse them.
It's a bit surprising that Bazel does not ignore `.git` out of the box
such that you have to opt-in to considering it rather than opting-out.