Resolver graph

[michaelpb]

• Fix currently hacked together resolvers.py system with a ConversionGrid based ResolverGrid singleton, that resolves a foreign URL by possibly changing between several steps

• Example: Git -> Cloned-git -> Checked out file .... OR Git -> Cloned-git -> git-tree.json... OR Git -> Cloned-git -> git-log.txt (timestamped)

converters/git.py
 10 class GitLsTreeToJson(converter.Converter):
 11     inputs = [
 12         'GIT',
 13         # TODO: Clean this up, use the detector system instead
 14         # ALSO TODO:
 15         # When finishing Resolver graph system, give option of popping out
 16         # known types in addition to unknown (for example, known type of
 17         # .git-ls-tree)
 18     ]

  6 # TODO:
  7 # * Presently hardcoded, refactor this into a "resolver" conversion system
  8 #    * Can use ConverterGraph except with Resolvers instead, which take a
  9 #    resource_url, and ensure that a ForeignResource is cached from that
 10 # * Replace with async subprocess stuff
 11 # * Possibly create async subprocess base class helpers used by both conversio
 12 # system and resolver system