linking scenarios.  We weren't searching for .a archives to satisfy
-lfoo options on the GHCi command line, for example.

I've tidied up the code in this module so that dealing with -l options
on the command line is consistent with the handling of extra-libraries
for packages.

While I was here I moved some stuff out of Linker.hs that didn't seem
to belong here: dataConInfoPtrToName (now in new module DebuggerUtils)
and lessUnsafeCoerce (now in DynamicLoading, next to its only use)