Check for specifying compiler program locations per-package

You're not allowed to specify a different compiler program for different
packages within the same project. Given that in general we allow
specifying config about programs on a per-package basis this needs a
specific check.

Also add a place and a TODO for Future work in this area which is to
check up front that we can find the programs and to monitor them for
changes. Also, currently none of the program config ends up in the
package hashes, so it doesn't cause them to be rebuilt.