refactor(downloader): take &self across the downloader hierarchy

Concurrent package operations call into the same downloader instances
through Rc<RefCell<dyn DownloaderInterface>>; with &mut self methods
every call holds a RefMut across its awaits, which panics with
'already mutably borrowed' the moment two operations overlap. This is
groundwork for fanning out InstallationManager's download/install
loops (same rework HttpDownloader/CurlDownloader already got).

- DownloaderInterface/ChangeReportInterface/ArchiveDownloader/
  VcsDownloader methods now take &self; as_change_report_interface
  returns &dyn instead of &mut dyn.
- Implementors move their genuinely mutable state behind cells:
  FileDownloader.additional_cleanup_paths, the archive downloaders'
  cleanup_executed, ZipDownloader.zip_archive_object,
  VcsDownloaderBase.has_cleaned_changes, GitDownloader's stash/discard/
  cache maps and GitUtil, SvnDownloader.cache_credentials,
  PerforceDownloader.perforce. FileDownloader.io gains a RefCell layer
  so get_local_changes can keep PHP's NullIO swap under &self.
- ProcessExecutor::execute_async now returns a future that captures
  everything up front instead of borrowing the executor, and call
  sites build the future before awaiting, so no borrow on the shared
  executor is held while a subprocess runs.
- Filesystem::remove_directory_async becomes remove_directory_async_via
  taking the Rc handle: the Filesystem is only borrowed for the sync
  head/tail, never across the rm subprocess await (sync borrow_mut
  users like rename/ensure_directory_exists would otherwise collide).
- DownloadManager async call sites hold shared borrows only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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