Use IFD to reduce copySources invalidation surface

[exFalso]

This PR fixes copySources invalidating the two-stage build. The reasons for the invalidation:

• copySourcesFrom was an attribute of the dummy-src derivation, causing invalidation if any sources changed
• copySourcesFrom was used directly as the cp source directory with $copySourcesFrom/$p, invalidating the command script even if $p has not changed

The PR introduces the following changes:

• $copySourcesFrom/$p changed to ${copySourcesFrom + "/${p}"}, and the surrounding for loop to a concatStrings. This causes the subdirectory to be treated like a proper nix path, so as long as copySourcesFrom is a local path, the script will only invalidate if $p changes.
• An IFD indirection was introduced in the form of ifdForceCopy. This is to handle cases where copySourcesFrom is not a local path, but a /nix/store one (like in our use case). The indirection effectively "snips" the dependency chain, so it doesn't matter what derivation copySourcesFrom comes from and how it invalidates, ifdForceCopy will re-copy the subdirectory as a separate "leaf" store path. NOTE: this solution will effectively re-copy the sources twice. This could be reduced somewhat by using gzip instead of straight copying for the IFD.
• copySourcesFrom was removed from dummy-src's attributes

[Patryk27]

Hi, thanks for this merge request - unfortunately, unless there were any recent developments in this area, to keep naersk working with Hydra, we have to avoid using IFD; and so given the choice, I'd rather keep the current approach; thanks again!