Eliminates the need to rebuild some source files if the file_util header
ever changes. This also uncovered some indirect inclusions, which have
also been fixed.
The follow-up to e2457418da, which
replaces most of the includes in the core header with forward declarations.
This makes it so that if any of the headers the core header was
previously including change, then no one will need to rebuild the bulk
of the core, due to core.h being quite a prevalent inclusion.
This should make turnaround for changes much faster for developers.
This doesn't actually modify the internal class state, so it can be a
const member function. While we're at it, amend the function to take
its arguments by const reference.
