Fix rust_analyzer flycheck label for sources in subdirectories

[brunal]

query_label_for built the saved file's label from its basename rather than its path relative to the package, so for a file at foo/bar/src/publisher.rs it queried

attr(srcs, "//foo/bar:publisher.rs", //foo/bar:*)

which matches nothing. Flycheck then failed with "bazel query returned no targets" and rust-analyzer reported "Cargo watcher failed, the command produced no valid metadata".

This problem can easily arise in cargo-compatible codebases, as the "src/" subdir is standard with cargo.

Note that crate root modules are exempt because build_file_label_map populates the sidecar's file_labels from each spec's root_module, so saving lib.rs or main.rs takes the fast path and never runs the query. Every other saved file in a package with a src/ layout hit this.

Pull the label construction into source_file_label/package_label so it can be unit tested, and normalize \ to / there -- both halves are derived from the saved file, so on Windows they arrive backslash-separated and //foo\bar:... is not a valid label. Implementation-wise, I simply always normalize, when there could be a platform-specific implementation.

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