Read `setup-config' into a strict ByteString, making sure that the
file handle does not stay open.

This caused permission denied problems on MSYS where the working
temp directory could not be removed after building a package because
of a lingering open file.

Additionally, I've uglified the qualifier for the import of
Data.ByteString.Lazy.Char8, since it's ugly and potentially risky,
and should be clearly visible as such.