Store results of package/executable lookups

[jimporter]

To help make more reproducible builds, we could keep track of the exact results of package/executable lookups (e.g. system_package()). This way, even if something about the environment changes, we'll still have consistent results (although sometimes this might just mean a build failure).

One open question is: where should this information be stored? Currently, I see .bfg_environ as independent of the build.bfg file, and so the results of lookups wouldn't go there. This implies that .bfg_environ is immutable even when build.bfg is edited, which is a potentially-nice property (keeps things simple). However, these results do technically come from the environment, so maybe they really do belong there.

[jimporter]

Closing; probably better to address in mopack: jimporter/mopack#1