rb-site install will now permit installing into a site directory that

contains only a venv folder. This is to enable deployments where a

virtual environment is first set up in the destination sitedir, and then

the installed rb-site is used to populate the sitedir.
There's also a --allow-non-empty-sitedir option for advanced use,

which bypasses the empty sitedir check.

Tested installing into a populated sitedir. It gave me the expected error.
Tested installing into a new location. Installation was allowed.
Tested installing into a directory containing just a venv directory.

Installation was allowed.
Tested installing into an empty directory. Installation was allowed.
Tested installing into a populated sitedir and using

--allow-non-empty-sitedir. Installation was allowed.