Specifying input files that are not in the current directory and without specifying --src-prefix has always had pretty broken results: the compiler can actually break out of the specified output directory when creating output files! This will soon no longer be allowed.

capnp-test, when run in an out-of-tree build, currently specifies such an unprefixed input, but got away with it because it was in the errors test, which doesn't actually output anything because there are input errors. We fix it by specifying a proper --src-prefix.