Following apkg commands got support for -O/--result-dir option to
override directory where apkg puts resulting files:

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

When result is cached elsewhere, it's automatically copied to
--result-dir.

This change required refactoring across apkg commands:

* all commands and scripts now return list of all output files
* all output files are cached
* make commands and their options consistent with each other and --help
* new apkg.lib.common module to contain common/shared code

Related: #9