It was combining the executable's hs-source-dirs with the library's
hs-source-dirs when pre-processing executables. This was inconsistent
with the search path used for finding ordinary .hs files and with
sdist and lead to confusing behaviour. See ticket #161.
If an executable needs source files from the library then it must specify
the right hs-source-dirs, including '.' if appropriate.
The behaviour is that if hs-source-dirs is not specified then the default
is '.' however if hs-source-dirs is specified then '.' is not implicitly
added. If it's needed then it must be added explicitly. This was always
the case for ordinary .hs files, this patch just brings things into line
for files that need pre-processing.