Save configured sdists with a standard filename #1430

Some supported source: configurations fail to download source, or save it under a name that later commands cannot find. This prevents the package from building.

Examples:

• A hook-sdist resolver hook returns a tag archive URL ending in /v1.0. With no download hook, the download fails with InvalidSdistFilename.
• pypi-download with download_kind: sdist saves pkg-v1.0.tar.gz. Later, find_sdist() searches for pkg-1.0.tar.gz and finds nothing.

Expected: Save the archive using the package name and resolved version, preserving .tar.gz or .zip. Both cases should work without a custom download hook. Add regression tests for both cases.

Observed on new-resolver-config at ff8d7683. Related: #1414 — review comment.