[entropy.client.package.actions] action: fix typo in path_lock()
[entropy.client.package] fetch: add initial concurrency support
[entropy.client.package.actions] fetch: remove unused method
[entropy.client.package.actions] action: move _stat_path() here
[entropy.client.package.actions] install: complete concurrency support

Add concurrency support to package files.
[entropy.client.package] rename fetch phase to _fetch_phase()
[entropy.client.package.actions] action: add stat import