Fix package-relative paths for external sources

[bduffany]

Context: this unblocks migration of BuildBuddy to rules_js. We build our app externally like bazel build @com_github_buildbuddy_io_buildbuddy//enterprise/app:app_bundle when deploying via our internal repo, and this is broken because the current logic in rules_swc results in files being written to the wrong path.

Before, we'd get an error from esbuild like app.tsx: ERROR: Could not resolve "./root/root", and inspecting the sandbox dir after building with --sandbox_debug, the tree would look like this (filtering just to app.tsx and root.js here which are the main relevant files):

$(bazel-sandbox)/buildbuddy_internal/
    - bazel-out/k8-opt/bin/
        - external/com_github_buildbuddy_io_buildbuddy/
            - enterprise/app/
                - app.tsx  # esbuild entrypoint
                - external/com_github_buildbuddy_io_buildbuddy/enterprise/app/root/
                    - root.js # generated by SWC

The problem is that esbuild can't locate ./root/root.js because it's nested under ./external/com_github_buildbuddy_io_buildbuddy/enterprise/app/root/ relative to app.tsx, when it should just be under ./root/root.

This PR fixes that issue. After the fix, the directory structure looks like this:

$(bazel-sandbox)/buildbuddy_internal/
    - bazel-out/k8-opt/bin/
        - external/com_github_buildbuddy_io_buildbuddy/
            - enterprise/app/
                - app.tsx
                - root/
                    - root.js

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[alexeagle]

Thanks!
I'd really like to keep our helpers consolidated in https://docs.aspect.build/rules/aspect_bazel_lib/docs/paths especially since I think this bug is present in many rules. @kormide FWICT we don't have exactly this function there already? Should we add a new one?

Also note that next week we'll land #57 and will stop using short_path because we won't run swc under the nodejs binding anymore, and can follow "normal" bazel pathing idioms.

[alexeagle]

Release v0.21.0 is published, I think that should solve this issue by removing the short_path logic altogether.

[bduffany]

Release v0.21.0 is published, I think that should solve this issue by removing the short_path logic altogether.

Nice! I tested with v0.21.0, and it looks like we still need the fix for _relative_to_package, since without it, the package-relative relative source path for @com_github_buildbuddy_io_buildbuddy//enterprise/app/root:root.js is computed as ./external/com_github_buildbuddy_io/buildbuddy/enterprise/app/root/root.js when it should just be ./root.js relative to the package. But I reverted the other parts of the PR which are no longer necessary.

Side note, v0.21.0 is blazing fast 🔥 I can't even see the SWC compile actions in the progress output after a clean build.