Document the splitoff of plugins

[DanBurton]

We moved some plugins, and this caused confusion. Whoops. :(
http://www.reddit.com/r/haskell/comments/3526x2/stackage_problems/

For users who wish to install via cabal, the recommended approach should be cabal install stackage.

We should (soon) encourage users to install via stackage-setup; the recommended approach for this should be:

• download executable
• ./stackage-setup
• possibly followed by a second command to alter the PATH appropriately (maybe append a line to $HOME/.bashrc or something)

[DanBurton]

I've added libraries with currently-empty modules to the packages that were executable-only. This stops cabal from complaining about executable-only dependencies, and also provides a place to librify some of the functionality in each executable.

The next step is to adjust our advertising from cabal install stackage-cli to cabal install stackage.

[DanBurton]

I added a post-install hook on stackage-cli that is supposed to print a message about how to get some of the plugins. However, for some reason, cabal isn't running this hook. haskell/cabal#2579

[snoyberg]

That bug is... unfortunate.

[DanBurton]

As part of better documenting plugins, we should also have a better error message than "invalid argument" for trying to call something like stackage sandbox when the sandbox plugin is missing. We can have a hard-coded list of known plugins and give more helpful error messages about how to get them.