Remove dev-requirements.txt files

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