WIP: use Meson instead of setuptools to install and distribute BuildStream

We have talked from time to time about getting rid of setuptools. The discovery of issue #172 (closed) motivated me to actually prototype this to see if we can actually do it.

This branch demonstrates a pretty functional Meson port of our setuptools build system.

Upsides:

• pkg_resources is only imported if your project chooses to find plugins installed with Pip, so we speed up the start time of bst
• the build system will never download or install dependencies for you
• setuptools_scm is used to decide on a version number, as before
• man page generation runs for each build
• configure-time options are supported (no need to pass in options as environment variables any more)
• easy to integrate C code into the project at a later date if we need to

Downsides:

• every .py file is manually listed in the meson.build file, instead of being discovered manually
• every man .1 output file is listed manually as well
• no --editable mode; you have to ninja install every time you make a change, or manually symlink the buildstream package dir from your source tree into your Python library path

Not yet implemented:

• sphinx documentation generation
• distribution tarballs: the ninja dist target breaks because setuptools_scm can't discover the version number when run as part of the dist process as it runs outside the source repo
• BST_ARTIFACTS_ONLY mode

Let me know what you think of this; I can look into fixing the remaining issues if the general approach seems sound.