Nicer error message when source files are not found.

[duncan]

It can be confusing sometimes when source files are not found. Eg:

$ runhaskell Setup.lhs build
Preprocessing executables for nm8-0.1...
Setup.lhs: can't find source for Data.MaybeI in src, dist/build/autogen
$ find src/ | grep Data
src//Data
src//Data/MabyeI.hs

In this case it's simply a spelling error, but it's still hard to spot initially.

The suggestion is to look look at all files in that directory and do an edit distance match and suggest any similar file. This would also help for case-sensitivity mistakes, see #521.