Following commands got support for automating cache and reuse of
produced output files:

* make-archive
* get-archive
* srcpkg
* build

Cache uses checksum of current project state as a key and also stores
and validates cached files' checksums. Currently the project checksum is
obtained using git magicks so the cache is only enabled for projects
using git.

Cache lives in pkg/.cache.json and it's updated automatically when
a new archive/srcpkg/package is created or when invalid cache entry is
requested.

Caching can be disabled using new --no-cache option.

Cache can be cleared by removing pkg/.cache.json file.

Self test on make-archive is included.