**Note** The first commit is from PR #416 and doesn't need to be reviewed here ------------ This commit adds a lifecycle callback that "prunes" (removes) those primed files that already exist (with same contents and permissions) in the base layer. This is necessary because the dependency resolver doesn't look into the base layer when deciding dependency cutoffs (and also to support chisel slices, which only look at the target install dir). Some notes: 1) This first implementation only looks are regular files (no symlinks and directories), as those are the simplest to handle and provide the biggest size "wins". If necessary, we can address symlinks and/or directories in follow-up work. 2) This could have been implemented during the archiving of the new layer, where we already loop on the files and look at the base layer (to handle things like the usrmerge). We decided to implement this as part of the lifecycle to improve the "clarity" and "debuggability" of Rockcraft runs: after running the lifecycle, the prime directory contains exactly what's going to be packed in the new layer. The resulting ROCK should be "semantically" equivalent, but smaller. For instance, the Pyfiglet ROCK in docs/tutorials/code/pyfiglet goes from 69MB to 53MB, with no functional changes. Fixes #363