There are now 3 cache targets along with associated config options:

* cache.local
* cache.remote
* cache.source

cache.source is only enabled when VCS is available, other cache
targets are unaffected by VCS and are enabled by default.

Caching apkg commands require following cache targets:

* make-archive: source
* get-archive: remote
* srcpkg: local (archive), source (templates unless --upstream)
* build: local (source package)

This leads to cache only being disabled for relevant commands working on
project source when VCS isn't available as opposed to disabling all
caching.

Logging was vastly improved to get useful cache information when using

    apkg -L verbose $COMMAND

Docs were expanded with a new cache.md as well as upated with clearly
worded requirements for working cache (VCS and proper make_archive_script).

Simple test on examples/minimal-no-git is included in
ci/tests/test_apkg.py for testing operation without VCS.

Closes: #73