Get the list of Reqs in the cli, and pass into
install_repository_specs_loops()

'editable' and 'namespace_override' are only needed for creating
the Requirement() objects, so dont pass into loop

install_repositories_matching_repository_specs() was no
inaccurate as well as still being too long, so change it.

ie, one InstalledRepositoryDatabase() instead of a new instance
in each helper method. The IRDB class is stateless, but using
the same ref makes unit testing easier.

Update unit tests for better coverage of action.install

kinda sorta emulate a dep solving iteration

Move requirements_from_strings() from ansible_galaxy.actions.install
to ansible_galaxy.requirements with from_requirements_dict

Reorder the methods in ansible_galaxy.actions.install based on
order of execution instead of the other style which we'll
call... aleatoric?

Add test_requirements unit tests

There is no longer the separate concept of 'dependencies'
(install time) and 'requirements' (run time... and install time).

There are only 'dependencies' now.

Which we call 'requirements' almost everywhere
inside the code because reasons.

But for the moment, at least consistently call
them by the wrong name everywhere, so they
can all be changed at the same time.

The ansible_galaxy_cli code now calls
ansible_galaxy.actions.install.run() instead of
install_whatever_loop() as before.

Plan is to move other actions to similar api.

A run() that more or less takes the cli args (strings).

And a suitably named method in the action module that
shows a more pythonic signature.

For example, here
run() takes a list of strings for requirement_spec_strings,
while install_requirements_loop() takes a list of Requirement()
objects.

Also, run() returns only a return code suitable for sys.exit(),
while the more pythonic method will return a results dict with
a 'status' and an 'errors' list.

Out of ansible_galaxy.install and into repository_spec
so it is easier to share.