Filter out Build_doctests (can't have doctests)

We used to only get module sources from library and executable components.  So
the `Build_doctests` module wouldn't show up.  But for issue ulidtko#57 we're now
getting module sources for the test suites as well, and Cabal nags us with the
warning:

    <no location info>: warning: [-Wmissing-home-modules]
	These modules are needed for compilation but not listed in your .cabal
        file's other-modules:
            Build_doctests

So people are likely to follow the warning and put in Build_doctests into their
Cabal file, and then be annoyed that it breaks their tests.

Because `Build_doctests` is autogenerated by Cabal-doctests, we know it will
never have doctests itself, so it can be safely filtered out.

src/Distribution/Extra/Doctest.hs

@@ -344,7 +344,8 @@ generateBuildModule testSuiteName flags pkg lbi = do
            -- pass the full path to the main-is module instead.
            mainIsPath <- T.traverse (findFileEx verbosity iArgsNoPrefix) (compMainIs comp)
 
-           let all_sources = map display module_sources
+           let all_sources = filter (/= "Build_doctests")
+                             $ map display module_sources
                              ++ additionalModules
                              ++ maybeToList mainIsPath