Fixes dependencies in Makefile.

Simplifies some rules to avoid redundant builds.
Changes Makefile and file search algorithm.

One test was repeatedly failing on Travis because
the imported file could not be found.  The Makefile was
also recompiling unnecessarily.   Some of the dialects
were not being built at all, and do not yet work.
Adds stubs.

dom.grace for the Web DOM, and imports.grace for the
mysterious "imports" module.