Major patch to fix reporting of unused imports

This patch, joint work between and Ian and Simon, fixes Trac #1074
by reporting unused import declarations much more accuratly than
before.  The specification is described at

http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/UnusedImports

The implementation is both easier to understand than before, and shorter
too.

Also fixed are #1148, #2267

Also fixed is -ddump-minimal imports, which now works properly, fixing
Trac #1792.