Combine search paths to avoid duplication.

Thanks for patiently walking me through this, I hope it's OK now. Locally it does correctly search for (and use) the profiled version of plugin libraries (e.g. libHS[...]_p.a).

Follow method recommended by Phyx to find plugin lib files.

Wasn't that what https://ghc.haskell.org/trac/ghc/ticket/15492 was about? I thought it showed that we should not be adding the _p suffix.
That might not be entirely true, but at least it shows that we don't want to add the _p suffix in all cases (in particular, not in the situation of that ticket). Doesn't that then mean that if we want to sometimes add _p, we would have to check for the WayDyn way to decide? I tried to avoid that as per your initial suggestion, but perhaps I misunderstood.

Thanks Phyx.

Simpler plugin location search, as recommended by Phyx.