Remove path resolution from internal forks plugin

Alternative to facebook#23254

Our build script has a custom plugin to resolve internal module forks.
Currently, it uses require.resolve to resolve the path to a real file
on disk.

Instead, I've updated all the forked module paths to match their
location on disk, relative to the project root, to remove the need to
resolve them in the build script's runtime.

The main motivation is because require.resolve doesn't work with ESM
modules, but aside from that, hardcoding the relative paths is more
predictable — the Node module resolution algorithm is complicated, and
we don't really need its features for this purpose.