Move the requirements files into a single directory

[python-desert]

The requirements files would be better off in a single directory to make them easier to see.

[altendky]

And the locks in a separate from the inputs. I haven't done this myself yet but want to. I haven't thought much about a sub-directory vs. an adjacent directory.

/requirements.in/base
/requirements.in/test
/requirements.in/dev
/requirements.txt/base
/requirements.txt/test
/requirements.txt/dev

/requirements/in/base
/requirements/in/test
/requirements/in/dev
/requirements/txt/base
/requirements/txt/test
/requirements/txt/dev

/requirements/base
/requirements/test
/requirements/dev
/requirements/txt/base
/requirements/txt/test
/requirements/txt/dev

At the moment I'm tending towards the last one.

When working in pycli I did have an itch to get away from the special case for base with had no qualifier or - (requirements.in) compared to test (test-requirements.in). It would be nice for every layer to have a name.

(where in and txt can be whatever, not meaning to pass any comment on that here)

[python-desert]

Let's call them 'lock' or something instead of 'txt'.