[bazel] Improve runfiles handling (#10098)

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

.bazelignore

@@ -1,3 +1,4 @@
 # Without this, Bazel will consider BUILD.bazel files in
 # .git/sl/origbackups (which can be populated by Sapling SCM).
 .git
+codex-rs/target