Moving fetch and track to the source command group accidentally dropped
the support for guessing targets for these commands when invoked from a
workspace directory. This brings it back.

_frontend/cli.py: Reinstate support for guessing targets

See merge request !1036

Chandan Singh authored Jan 03, 2019 and Tristan Van Berkom committed Jan 04, 2019

This seems like a better name for the directory, as it more closely
describes the purpose of its contents.

Chandan Singh authored Jan 03, 2019 and Tristan Van Berkom committed Jan 04, 2019

Split the "The MANIFEST.in and setup.py" section in two:
"Managing data files" and "Updating BuildStream's Python dependencies".

Briefly explain the layout of `requirements` directory and add
instructions to use the Makefile added in the last commit.

Add Makefile to update requirements files

See merge request !1035

As we now run tests using `tox`, we don't need to worry about manually
packing and unpacking BuildStream. So, we can remove the preapre stage
entirely.

Update `coverage` and nightly jobs to appropriately cope with this
change.