Rollup merge of #119582 - arlosi:bootstrap-vendor-remap, r=wesleywiser

bootstrap: handle vendored sources when remapping crate paths

#115872 introduced a feature to add path remapping for crate dependencies, but only when they came from Cargo's registry cache, not a vendor directory.

This caused builds that used remapped debuginfo and vendor directories to fail with:
```
std::fs::read_dir(registry_src) failed with No such file or directory (os error 2)
```
or (if the `registry/src` directory exists but is empty)
```
error: --remap-path-prefix must contain '=' between FROM and TO
```

Fixes #117885 by explicitly supporting the `vendor` directory and adding it to `RUSTC_CARGO_REGISTRY_SRC_TO_REMAP`.

Note that `bootstrap.py` already assumes that `./vendor` within the rust repo is the only supported vendoring location.

r? `@pietroalbini`

Author: matthiaskrgr