FindPythonModules: use a function

Prevent unnecessary scope pollution.

Change-Id: If268d5fcbcf4edd7b5dcac62fa567ca4cf634082
FindPythonModules: search if not found

It might have been installed to satisfy the module requirement since the
first check.

Change-Id: I1775c3d060cbf75bec383309fd026a4cf7b348a3
Merge topic 'python-find-module' into master

9b2bba13 FindPythonModules: search if not found
a2f0bf4d FindPythonModules: use a function