This is part of a later plan to implement mirroring without forcing
everyone to update their sources. We use the expected calls to
Source.translate_url() when running Source.configure() to extract the
aliases from the URL. Multiple aliases must be extracted because
sources exist that may fetch from multiple aliases (for example, git
submodules)

Later, we want to substitute another URI where the alias normally reads
from the project - We accomplish this by re-instantiating the Source
with the alias overrides passed as an argument to the constructor.

**KLUDGE WARNING**: This involves making the source store its "meta"
object so that it's possible to create a copy of the source inside the
fetch queue, instead of back when the pipeline was being loaded.

This adds the SourceFetcher class, which is intended for sources that
fetch from multiple URLs (e.g. the git source and its submodules)

Fix when fetching

In user config (buildstream.conf), it is set with the "default-mirror"
field.

On the command-line, it is set with "--default-mirror"

This fixes:
* Bzr repositories pulling from the branch they were created with.
* Bzr's _ensure_mirror() not actually checking that it successfully
  mirrored the ref.

When implementing fetching from mirrors, I encountered some problems
with the git source:

1. The mirror URL was using translate_url()'s output, so if a different
   alias was used, then fetching from the mirror would go to a different
   directory, and be inaccessible.
2. After fixing that, fetching was unable to pull from a URL other than
   the one used at repository creation, meaning it wouldn't actually
   pull from the mirror.

This is helpful if you want to test what happens when you have one repo
that has diverged from another. By copying the repo you're sure they
start with shared history.

This is especially useful when mirroring.