feat(vcs): wire cache-vcs-dir setting through to drivers

DriverConfig used to expose a generic cache_dir hardcoded to a
relative ".cache/mozart/vcs", with each driver appending its own
"git"/"hg" subdir. As a result, the cache-vcs-dir setting (and
COMPOSER_CACHE_VCS_DIR env var) had no effect on where mirrors
were actually stored.

Replace cache_dir with cache_vcs_dir, resolving its default the
same way Composer does (COMPOSER_CACHE_VCS_DIR → COMPOSER_CACHE_DIR/vcs
→ XDG/HOME fallbacks), and have GitDriver/HgDriver use it directly.
This brings the Mozart cache layout in line with Composer's: a single
shared vcs root with one subdirectory per sanitized URL.

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