profile_start() and profile_end() calls have been incorporated into
Scheduler.run()

profile_start() and profile_end() calls have been added to
Stream.load_selection()

Add new 'scheduler' and 'load-selection' profiling topics

See merge request !1088

Previously, one had to manually install `click-man` package, and
remember to run the correct command. Now, we can simply run `tox -e man`
to update the man pages.

Notable changes:

* New `source` and `artifact` command groups
* Man pages corresponding to obsolete commands, that were marked as
  hidden in Click, have now been removed.

Fixes #881.

Simplify our docs, by requesting users to run `tox -e man` to update man
pages, instead of manually installing `click-man` and running the
command manually.

If the `man` directory is empty, then it won't be copied in the source
distribution, and `list_man_pages()` will throw an exception when trying
to list files in a non-existent directory. This prevents us from
installing the BuildStream package when the man pages are not there.

The most common use-case for this is when we want to re-generate the man
pages but want to install the package before re-generating them.