Add --debug-workspace

Avoids Package#cleanup calls and also prints the paths, if not empty, of
any build or staging directories used for input or output.

Fixes #720
Don't install dependencies when using pip to download python packages
Show full input path if it can't be found.

Before: Cannot package the path 'pganalyze-collector', does it exist?
After: Cannot package the path './build/pganalyze-collector', does it exist?
Merge pull request #727 from dknecht/master

Don't install dependencies when using pip to download python packages
Merge pull request #734 from jordansissel/add-debug-workspace

Add --debug-workspace