Properly handle directories with an "install" tag.

The current way of installing directories is overly simple: We just do a recursive copy with no knowledge of what's in the subtree. This is problematic, because we are not able to track changes inside that subtree, meaning newly added files in there will not be installed during subsequent builds. There are two ways to fix that problem:
a) The simple solution. Always treat directories as "dirty", i.e. execute the install rule every single time to be on the safe side. This has potentially prohibitive cost, since the data in there can be arbitrarily big.
b) The correct solution. Perhaps this can be as simple as using a "recursive timestamp" for directories.