Rename requires files to standard names.

Rename tools/pip-requires to requirements.txt and tools/test-requires
to test-requirements.txt. These are standard files, and tools in the
general world are growing intelligence about them.

Fixes: bug #1179008
Change-Id: I6f8a11988982133249d443403f63b4f8724e156e
Merge "Rename requires files to standard names."