Reuse Git sources fetched by download-sequence #1431

Prefetching Git sources does not allow a later build to run offline. The build tries to clone again even though the required checkout is already present.

Run download-sequence, then build-sequence using the same settings and work directory, without a cached wheel:

• With cleanup enabled, the existing checkout is deleted and cloned again.
• With --no-cleanup, the build raises FileExistsError.

Why fix it: A build without access to the source host fails despite having prefetched its sources. Online builds also repeat the download unnecessarily.

Expected: Safely reuse a matching, clean checkout. Add coverage for offline replay and --no-cleanup.

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