Add `.in` and `.txt` requirements files for BuildStream's pure python
dependencies. For each pair, the `.in` file is supposed to capture the
loose version requirements, and the corresponding `.txt` file is
supposed to have frozen requirements. We have 3 such sets:

* `requirements`: BuildStream's runtime dependencies
* `dev-requirements`: Dependencies for running tests
* `plugin-requirements`: Dependencies for core plugins

Note that the frozen requirements files will only be used for testing
purposes, and `setup.py` will continue to read loose requirements.