Allow discovering more fields

[tfausak]

Essentially blocked by #66.

Currently Gild's discovery mechanism only works for three fields: exposed-modules, other-modules, and signatures. I think there are other fields where it would make sense to support discovery:

• license-files
• data-files
• extra-source-files
• extra-doc-files
• includes
• install-includes
• c-sources
• cxx-sources
• asm-sources
• js-sources

Note that they discover files rather than modules, so they're a little different than the existing ones.

Probably shouldn't support extra-tmp-files, since the intent is to list files that can be deleted. That means running Gild will likely produce different results depending on if the directory is clean or not.