fix: add support for `--incompatible_use_plus_in_repo_names`

It was [found][1] the `~` character slows down C/C++ includes on Windows.

Bazel 8 will change to the `+` symbol.

The [--incompatible_use_plus_in_repo_names][2] flag allows testing this early.

This patch supports any separator.

[1]: https://github.com/bazelbuild/bazel/issues/22865
[2]: https://github.com/bazelbuild/bazel/issues/23127