rust-analyzer discover_bazel_rust_project invalid package separator on Windows

[AlexHickie]

When using @rules_rust//tools/rust_analyzer:discover_bazel_rust_project, the paths are assumed to have / separators 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 '\')