Ensure packaged artefacts are fully replaced in dist/

[huonw]

This patch has the package goal clear out the artefact(s) it is about to write to dist/, if they already exists.

For instance, if running pants package path/to:target on a pex_binary (outputting path.to/target.pex), pants will now first remove anything that's already at dist/path.to/target.pex before writing the new output.

This resolves two problems:

• if there's existing contents of a different kind (e.g. a directory in dist/ and writing a file), the package call would explode. For instance, switching a target like pex_binary(..., format="zipapp") (file) to pex_binary(..., format="packed") (directory).
• if the package output is directory, stale files already in that location in dist/ would remain. For instance, a pex_binary(..., format="packed") where a file was removed.

This fixes #17758 and fixes #18849, respectively.

This only fixes package, not any other goals that also write to fixed paths (like export and export-codegen). In #18871, I start on export-codegen, but it's a bit fiddlier (requires propagating "this is the artefact" paths around) and it's best to land the infrastructure in this PR first. I'll file follow-up issues covering them specifically.

Commits are individually reviewable.

[stuhood]

Thanks!

[huonw]

I'll file follow-up issues covering them specifically

• for export-codegen: export-codegen is not resilient to existing contents in dist/codegen #18941
• export is already 'safe':

  pants/src/python/pants/core/goals/export.py

  Lines 147 to 150 in a24a59e

  	output_dir = os.path.join(str(dist_dir.relpath), "export")
  	for result in flattened_results:
  	digest_root = os.path.join(build_root.path, output_dir, result.reldir)
  	safe_rmtree(digest_root)