When building in Bazel actions, notably building runtimes, using
absolute paths makes the results non-hermetic and generally less
cache-friendly.
This restructures the code to only form an absolute path as part of the
`bazel run` change of working directory. It also tries to make the API
for doing this a bit more clear by taking the `exe_path` and
transforming it internally.
To support this, this PR also generalizes the `RemovingDir` to support
relative paths. While these can be tricky -- the working directory needs
to not change while they exist -- that isn't a reason to fully exclude
them and they're useful for implementing relative-path runtimes, etc.
1) toolchain and explorer use the same working dir logic, share it
2) explorer's carbon.cpp and main_bin.cpp use the same relative path logic, share it
3) You can append a longer path in one call with the right kind of iterator
4) Fix what's maybe a bug in passing `relative_prelude_path.str()` to `cl::init`
5) Collapse Main and ExplorerMain to avoid passing more parameters between
Chandler Carruth
Jon Ross-Perkins