21601: Build Python distro packages from wheels

21601: Build Python distro packages from wheels

The immediate problem this solves is that, by building and installing
from a repository of wheels, Python packages can find their
interdependencies without any special logic in the build process.

Other benefits:

* Eliminates some redundant work. We don't have to build the Python SDK
  from source multiple times. We can use the published cwltest wheel
  instead of building our own.

* Prepares the code for PEP 517 compliance. We only invoke setup.py to
  build packages that have not been updated yet. We introspect packages
  from their wheels, so we no longer have to introspect the source to
  build distro packages.

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