look for "unresholved" paths

[abathur]

resholve's Nix API uses two separate builds:

1. an inner build to let the shell project build its ~final source normally (in case for example its Makefile does any dynamic templating/generation/etc.)
2. an outer build that just operates on and resholves the output of the first

This means that it's possible for something like the patchPhase to inadvertently introduce references to the unresholved output.

I'm not sure I want resholve itself to cope with these (since they're Nix-specific), but:

• it might make sense for the Nix builder(s) to grep for them
• it might make sense to be able to give resholve a list of forbidden strings, which the Nix API could do

Leaning towards grep?