Save to buildinfo the library resolution of all "use"d package.

We call these the pluginProviderPackages because these are the packages that, if
they change, could affect the set of plugins available to this package (and thus
require it to be rebuilt).
Rebuild a package when its dependencies resolve to a different path.

Also simplify a bunch of library code that thought it might have to rebuild
warehouse packages, now that the warehouse contains unipackages.
BUILT_BY bump
document and rename pluginProviderPackageDirs