Carefully check for site packages without import.
Carefully check for site packages without import.

Tango app building must not hide ImportErrors which are present in the
site package.  Supporting flexible command-line parsing however requires
an import check to allow both implicit and explicit site package names.

This supports both `tango subcommand default` and
`tango subcommand tango.site.default` which allows for conventional and
unconventional tango site packaging, in terms of Tango conventions.
This is Python, and we intend to support it.

willowtreeapps/tango-core