Add ghc-mod command find

Hi Kazu,

I've added a find command to ghc-mod, which finds the modules where the given symbol is defined.

To be able to reuse most of the code of 'list' and 'browse', I've added two more low level functions
'listModsInternal' and 'browseInternal'.

The "right" thing to do here, that commands can easily reuse code of other commands, might be, that
all commands operate only in the Ghc monad and all this Cradle/withGhc stuff should be done above
all commands.

Currently the command can take quite a few seconds on bigger projects, where most time is spend
in allocating memory. Perhaps you've some ideas how to optimize it.


Greetings,
Daniel