[SV 54161] Fix second expansion of $* for paths

If the stem matches a path containing a directory not just a
filename, make sure the second expansion of $* in the
prerequisites matches $* in the recipe.  This requires using
$(*F) when replacing % in the first expansion to preserve the
simple filename.

* src/implicit.c (pattern_search): If lastslash is set prepend
the directory onto the stem.  Then use $(*F) when expanding %.
* tests/scripts/features/se_implicit: Add a test case