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. Both these jobs now install all runtime dependencies from
requirements files.

.gitlab-ci.yml: Remove prepare stage

See merge request !1037

Previously, we would put all jobs in the wait queue, if they were
not skipped. However, if a job is ready, its state will not change
in the future, and it is therefore useless to check its state
right after again.

This introduces a new `ready_queue`, which contains those jobs that
are then sent first.

Previously, we would update the state of all elements in the pipeline,
everytime an element in the queue would change state.

This is inefficient. We know that the only impacted elements will
be the direct build reverse dependencies. We can therefore, by recording
the direct build reverse dependencies, only update those and reduce
the time we spend updating states.