Library packages don't install in src/

I have a fix for this, but I'm writing this issue so I can reference it later.

Garden currently installs subpackages into the root directory of a package. This works because the only packages that currently use other packages are the repl and parsekit. Neither of those is a library (the repl script and the doc/bind scripts are all at the root of the package dir).

This IS a problem for testkitcore which is a library and depends on spawni and introspect. The fix involves also updating the catalog to mark non-library packages that have subpackages (easy and already done).