Pass download_kind to prepare_source overrides and document the archive name for source: packages #1442

From the #1414 review: #1414 (comment)

Two parts of the plugin contract change when a package with plugin hooks moves to source:, and the plugin isn't told.

1. prepare_source overrides don't know the source kind

sources.prepare_source() receives download_kind, but doesn't pass it to prepare_source overrides. With a *-git provider, an override written for archives receives a Git checkout directory and can't tell.

Proposed fix: pass download_kind to the override. Existing hooks are not affected, because overrides.invoke() drops keyword arguments that a hook doesn't accept.

2. expected_source_archive_name must return the new name

Configured downloads save archives as {canonical name}-{version} (#1438). If an expected_source_archive_name hook returns another spelling (for example zope_event-1.0.tar.gz), find_sdist() looks only for that exact file and doesn't find it.

Proposed fix: document the canonical archive name for hook authors using source:. Optionally, fall back to the usual spelling variants when the hook's file is missing.

Scope

This only affects packages that have these hooks and use source:. Packages without source: are unchanged. It is not a blocker for #1414.