Filter plugin dylib locations

Summary:
Previously we just created a cartesian product of the library
paths of the plugin package and the libraries of the package.
Of course, some of these combinations result in a filepath of
a file doesn't exists, leading to #15475.

Instead of making `haskFile` return Nothing in case a file
doesn't exist (which would hide errors), we look at all the
possible dylib locations and ensure that at least one of those
locations is an existing file. If the list turns out to be
empty however, we panic.

Reviewers: mpickering, bgamari

Reviewed By: mpickering

Subscribers: monoidal, rwbarton, carter

GHC Trac Issues: #15475

Differential Revision: https://phabricator.haskell.org/D5048