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.

Change-Id: I68ece8406fb1d4e082a42db8e76e17b1aaa7e775
Fixes: bug #1179008
Merge "Rename requires files to standard names."