model-checker: Kill dependencies and move back to libpspp.

Commit 95b074ff3 "Moved the datasheet testing code out of
src/{libspp,data}" moved the model-checker implementation from libpspp
into language/tests because it depended on math/moments.h and
data/val-type.h, which violates the dependency structure of the PSPP
libraries.

However, now I want to use the model checker in a test that should not
need to use anything from language/tests, so this commit eliminates these
dependencies and moves the model checker back to src/libpspp.