Summary:
Remove `dev-requirements.txt` files in favor of "test" extras_require.

We still had various mixtures of `requirements.txt`, `dev-requirements.txt`, and `setup.py` dependency constraints scattered across the codebase. This diff adopts the convention that all test requirements are specified under the `test` key of `extras_require` for our packages, so that you can look at a single file and see all dependencies for a package.

I also updated the dagster package creation template to not create requirements files anymore.

Test Plan: bk

Reviewers: rexledesma, dgibson, max

Reviewed By: rexledesma

Differential Revision: https://dagster.phacility.com/D6035