Wire prepare_source and build_sdist for git-clone-based source resolvers #1205

Context

When a package uses the new source: config with a git-clone-based resolver (github-tag-git, gitlab-tag-git, pypi-git), the resolve and clone steps work correctly via source_resolver.resolver_provider() and source_resolver.download(). However, the prepare_source step doesn't yet handle the cloned directory.

Parent epic: #1254 (Phase 4)
Depends on: Phase 3 (download dispatch — #1293)

prepare_source crashes on git-clone directories

sources.prepare_source() decides how to prepare source based on req.url:

if req.url:
    source_root_dir = pathlib.Path(source_filename)
    prepare_new_source(...)
else:
    prepare_source_details = overrides.find_and_invoke("prepare_source", default_prepare_source, ...)

When a package uses the new source config with a git-clone-based resolver, req.url is None, so it takes the else path. That calls default_prepare_source() -> unpack_source(), which tries to open source_filename as a .tar.gz or .zip. But source_filename is a directory (the cloned git repo). This crashes with:

ValueError: Do not know how to unpack source archive /path/to/mypackage-1.0

Fix approach

The download step (Phase 3) returns (path, DownloadKind) from source_resolver.download(). Use DownloadKind.git_checkout to detect when the source is a directory:

if download_kind == DownloadKind.git_checkout or source_filename.is_dir():
    source_root_dir = pathlib.Path(source_filename)
    prepare_new_source(...)
else:
    # existing unpack logic

Related

• build_sdist uses tarball instead of PEP 517 #1206 — companion issue for build_sdist() handling