The cache files of all our inplace dependencies which,
when updated, require us to rebuild. See #4202 for
more details. Essentially, this is a list of filepaths
that, if our dependencies get rebuilt, will themselves
get updated.
Note: the hash of these cache files gets built into
the build cache ourselves, which means that we end
up tracking transitive dependencies!
Note: This tracks the "build" cache file, but not
"registration" or "config" cache files. Why not?
Arguably we should...
Note: This is a bit of a hack, because it is not really
the hashes of the SOURCES of our (transitive) dependencies
that we should use to decide whether or not to rebuild,
but the output BUILD PRODUCTS. The strategy we use
here will never work if we want to implement unchanging
rebuilds.