If a project shows up in zuul's required-projects list, add it
to LIBS_FROM_GIT automatically. This way, when a user specifies
that a job requires a zuul-project, it gets used in testing, but
otherwise, it doesn't (pypi is used instead).
Also add information about what happens behind the scenes for both
LIBS_FROM_GIT and plugin dependencies.
This moves the check performed in check_libs_from_git to
a helper function which is installed for most kinds of
installations. This means that if someone sets LIBS_FROM_GIT to
"foobar", devstack won't error anymore, as nothing is going to
try to install foobar, therefore the check won't run on that.
However, as we move to automated generation of the local config,
that error is not likely to happen. This check was originally
added due to an error in the upper-constraints file (where a
constraint name did not match a package name). This location of
the check would still catch that type of error.
Change-Id: Ifcf3ad008cf42d3d4762cfb3b6c31c93cfeb40db
e1edde38ed