Implicit rules cannot invoke other implicit rules

If an implicit rule has an input that does not yet exist but could be made by another implicit rule, it will be skipped.

```
all: test.a

%.a: %.b
echo $< > $@

%.b:
echo $* > $@
```

This test makefile will successfully create `test.a` when run by GNU make, but ckati will fail with `No rule to make target 'test.a'` due to the pattern being declined because `test.b` does not already exist.