Re: Skip polyrec, etc. when templating from a sample

[Chris Warburton]

We now strip these names out before generating template values.

This required a little upheaval, since we were operating on Nix lists of
Nix strings, but doing the removals required running some 'runCommand'
calls.

Since we don't want to trigger a massive load of building during
evaluation, I've switched everything over to taking sampled names from a
file (possibly the result of a derivation) instead.