Fix regression in pip.installed

This fixes a regression introduced in #6574, where using the
'requirements' or 'editable' arguments would result in a traceback due
to a quoted empty string ('""') being passed to the call to pip.install.
Better state output for requirements/editable

This commit makes the output for these flags more accurate.
Merge pull request #6603 from terminalmage/pip-requirements-regression-fix

Pip requirements regression fix