Add a pm-projectinfo (pmpi) command

pm-projectinfo displays information on the projects available on the current Drupal site, including the list of extensions they contain (the reverse mapping of pm-info) and the Drush commands they define, if any.  If no projects are names, all are displayed.  With the --drush option specified, only projects that define Drush commands are listed.

Only projects that are part of a Drupal site are considered by this command.  Standalone Drush extensions are not visible.

This command came up as part of a discussion with Tom (I think it was) in Wellington.  He needed to be able to map from a project name (already downloaded) to the modules it contains.  While it seems that his actual use case might be better served by an improved pm-updatecode command (that respects module dependencies during partial upgrades -- maybe condtionally?), it does all the same seem to be really useful to have the inverse function for pm-info available.