Make caching more fine grained

Previously we only had a cache for the project info and each unit
info. However adding support for passing overridden compiler paths to build
tools introduces a nasty data dependency: to fully configure 'Program's
we (used to) need ProjInfo which needs an already configured 'Programs' in
readProjInfo (ugh).

After at least four failed attempts at untangling this I arrived at this
solution. Simply splitting up the caches into some smaller parts does the
trick and as a side product forced me to add an abstraction for the caching
logic so as to not reapeat myself even more.

Relatedly runQuery is not just a field accessor anymore but actualy does
some IO of itself to manage the cache and make already configured
'Program's available to the rest of the library.

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