Also highlight the fact that the plugin can rely on the fetcher's
fetch() method getting called before consuming the next item in the
list, which is the magick behavior that the git plugin relies on.

This is a part of #620

A download URL must be interpreted by the core at `Plugin.configure()`
time, even if only employed later on.

This is a part of #620

Keeps track of whether the plugin is currently being configured.

Adjusted Element and Source classes to call _configure() in place
of calling configure() directly.

This is a part of #620

The Source must now mention whether the marked or translated
URL is "primary" or not. Even when a Source may have multiple
URLs, the auxilliary URLs are derived from the primary one, not
only is this true for git, but it is mandated by our tracking
API which assumes there is a primary URL.

This adjusts the `git` source and the test `fetch_source.py` source
to behave properly and advertize it's primary URL properly.

This is a part of #620

This cannot test for unaliased URLs, as those can be discovered later
on outside of user provided element configuration; at least we
assert that if an alias was used, we have seen it at load time.

This will cause a BUG to occur for a plugin which marks an aliased
URL (or attempts to translate one) outside of `Plugin.configure()`,
if that URL was not previously seen.

This is a part of #620

Source fetcher changes

See merge request !772

Add a helper function assert_ref_in_track to git.py GitMirror()
which is used when staging & initing the source workspace. It
checks the element's ref exists in the track (branch/tag) if it
has been specified, raising a warning if necessary. The warning makes
use of the warning token 'REF_NOT_IN_TRACK' from the configurable
CoreWarnings. If the element has been tracked with bst, it is assumed
that the value of ref exists in the track as it was generated from it
& as such is not asserted.

Add tests that cover assert_ref_in_track & the configurable
CoreWarnings REF_NOT_IN_TRACK warnings token.

plugins/git.py: Warn if ref is not in given track

See merge request !564

For symmetry with _create_tree.

Track number of bytes required to commit or pull an artifact and return
this data on pull() and commit(). This takes deduplication into account.

The ArtifactCache.commit() return value is more accurate for what
__artifact_size is being used for, as it takes the deduplication done by
the CAS cache into account.

Before this commit, the cache size was being calculated every time an
artifact was committed to or pulled to the artifact cache. This is a
very IO demanding operation that doesn't scale as the cache gets bigger.

Now that the commit() and pull() methods of the ArtifactCache return the
number of bytes added, we can rework the code to calculate the size of
The cache only once at startup and to dynamically update it afterwards.

The ArtifactClean.clean() is also changed to return the number of bytes
removed instead of the new size of the cache, so that we no longer need
the ArtifactClean.clean._set_cache_size() method.

The CacheSize job is no longer used and is removed.

Fixes #573.