20846/21230: Modernize virtualenv setup in package building

20846/21230: Modernize virtualenv setup in package building

This change sets up the virtualenv earlier, then does all work inside
it, starting with installing build dependencies, to comply with PEP 668.

A couple optimizations that were easy to do as part of this work:

* We install modules in as few pip commands as possible, to reduce
  the overhead of determining what's currently installed, what's
  available from PyPI, etc.

* Removed a special case for CentOS 7 that we no longer need.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>