Try renaming `shim/` to make it more clear that it's an internal/infra concept

[dbort]

As described in #6860, the shim/ tree exists to make the buck2 build system work in both Meta internal and open-source repos. But no-one should really be looking at it most of the time, and it's more of a config thing than a part of the core tree.

We should see if we could rename it to something like .shim or build/buck2-shims. This would make it more clear that it's an internal config thing.

The main trick is that we need to coordinate this with the Meta-internal code mirroring config when we make the change.