ebuild: fetch: Factor out _get_uris

ebuild: fetch: Factor out _get_uris

The current fetch() function is quite long, which makes it hard to
know what I can change without adverse side effects.  By pulling this
logic out of the main function, we get clearer logic in fetch() and
more explicit input for the config extraction.

This block was especially complicated, so I also created the helper
functions _get_file_uri_tuples and _expand_mirror.  I'd prefer if
_expand_mirror iterated through URIs instead of (group, URI) tuples,
but we need a distinct marker for third-party URIs to build
third_party_mirror_uris which is used to build primaryuri_dict which
is used way down in fetch():

  if checksum_failure_count == \
      checksum_failure_primaryuri:
      # Switch to "primaryuri" mode in order
      # to increase the probablility of
      # of success.
      primaryuris = \
          primaryuri_dict.get(myfile)
          if primaryuris:
              uri_list.extend(
                  reversed(primaryuris))

I don't know if this is useful enough to motivate the uglier
_expandmirror return values, but I'll kick that can down the road for
now.

There was some discussion on the list [1] about the defaults in:

  def _get_uris(uris, settings, custom_mirrors=(), locations=()):

but I prefer this to Alec's floated:

  def _get_uris(uris, settings, custom_mirrors=None, locations=None):
      if not custom_mirrors:
          custom_mirrors = ()
      if not locations:
          locations = ()

Which accomplishes the same thing with less clarity ;).  My default
values are tuples (and not mutable lists) to make it extra-obvious
that we're not relying on anything crazy like mutating our default
values ;).

There was also discussion about whether the ugly settings object
should be passed to _get_uris, or whether the appropriate settings
should be extracted first and then passed through [2].  I've passed
settings through, because I'd prefer to have the uglyness handled in
helper functions.  The fetch() function is what most folks will care
about, so we want to keep that as clean as possible.  If the helper
functions have to suffer a bit for a cleaner fetch(), so be it.

[1]: http://thread.gmane.org/gmane.linux.gentoo.portage.devel/4002/focus=4041
[2]: http://thread.gmane.org/gmane.linux.gentoo.portage.devel/4002/focus=4042