Optimization: track actual file dependencies

Pre-mature optimization; recording for later consideration.

If we track which files are actually being accessed from the staged build dependencies, then we can make a more detailed decision whether we need to rebuild when a dependency changed. e.g. when project a.bst depends on project b.bst, but only on file /usr/include/b1.h, then when other files change in artifact(s) of b.bst should not trigger a rebuild. This data would obviously only be available once a.bst has been built once.

Potentially implementable by a fuse layer that either keeps track of first use, or that implements staging on-demand. The latter likely complicates the overall architecture. Avoiding rebuilds at the expense of regular build performance will need to be kept in mind.