Revisit how source maps are installed

[NTillmann]

Since #2362, we allow for multiple source maps to be given. However, we also require that the basenames of source file and source map file match.

So, instead or in addition to supplying a list of file names, we could allow suppling one or many directories in which we'll look for matching source map files.

Or alternatively, the arguments given to Prepack could explicitly map individual source files to then arbitrarily names source map files.

Discuss...