split into multiple package[.el]s

* Create separate packages for `magit-stgit', `magit-svn', and
  `magit-topgit'.
* Declare `magit-flow' as part of the `magit' package for now.
  The Melpa package does not depend that file though; it is not
  useful yet.
* Declare `magit-wip as part of the `magit' package for now.
  While it currently depends on the `git-wip' script we might
  remove that depency.  Keep it inside `magit' for now so we do
  not later remove a (potentially) tempory `magit-wip' package.
  If we decide not to remove the dependency on the script then
  we will create a separate package.
* Add "Package" header keyword to all libraries.
* Add "Keywords" and "Package-Requires" header keywords to all
  "main" libraries.
* Add "vc" (in addition to "tools") to "Keywords" header keyword.