RFE: automatic cache reuse

As discussed in apkg build MR, apkg already caches outputs of individual steps (archive, source package) in pkg/ so these should be reused automatically when no changes were introduced to the project since their creation.

Following needs to be implemented:

a check for project changes

apkg needs to know if project files changed since last package/source package/archive creation.

I'm thinking about adding a function to get current project checksum:

project.checksum()

This would work differently for git-managed repo and for projects without VCS (such as unpacked upstream tarball) so a new interface should be introduced for this along these lines:

vcs
├── git.py
├── hg.py
└── plain.py

apkg would autodetect VCS used and compute project checksum using respective vcs.* module. For projects without VCS, this would be done by checksuming entire project dir but excluding pkg/:

# this but without pkg/
$ tar c . | md5sum                                                                                                                                                                           
d0bdfdcebea3460de4a24062902b0669  -

save current project checksum and output filenames when creating archive/source package/package

On each successful creation of an archive, a source package, or a package, resulting output filename and current project checksum need to be saved.

Suggested locations:

pkg/cache/archives
pkg/cache/srcpkgs
pkg/cache/pkgs

reuse cached files when project checksum didn't change

make-archive, srcpkg, and build should skip build and return cached files if the project checksum didn't change since their creation.

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

Cache can be cleaned by deleting pkg.