[lit] Split TestingConfig.frompath() into separate ctor and load methods.
[lit] Inject the lit specific config object as 'lit_config' when loading config files.

 - Injecting it as 'lit' is gross, since that name should be used to refer to
   the actual package. For now both are available so it is possibly to cleanup
   test config files incrementally.
[lit] Rename lit.{TestFormats,Util} to their aliased names {formats,util}.

 - With compatibility hack in lit.__init__, so this hopefully shouldn't break
   anything.