rust-analyzer discover_bazel_rust_project invalid package separator on Windows

When using `@rules_rust//tools/rust_analyzer:discover_bazel_rust_project`, [the paths are assumed to have `/` separators](https://github.com/bazelbuild/rules_rust/blob/a4353c8fdfadc3bbf16af300e6ad19d0e33fb986/tools/rust_analyzer/lib.rs#L189) which isn't true for Windows.

```
Project discovery failed: bazel build failed: (exit code: 1)
INFO: Invocation ID: 503d9deb-2a50-4d29-ab71-3226769bed24
Computing main repo mapping:
Loading:
Loading: 0 packages loaded
WARNING: Target pattern parsing failed.
ERROR: Skipping '//foo\bar\baz:all': invalid package name 'foo\bar\baz': package names may contain A-Z, a-z, 0-9, or any of ' !"#$%&'()*+,-./;<=>?[]^_`{|}~' (most 7-bit ascii characters except 0-31, 127, ':', or '\')
ERROR: invalid package name 'foo\bar\baz': package names may contain A-Z, a-z, 0-9, or any of ' !"#$%&'()*+,-./;<=>?[]^_`{|}~' (most 7-bit ascii characters except 0-31, 127, ':', or '\')
```