package: check api call argument types, and improve LDocs.

* specs/package_spec.yaml (find, insert, mappath, normalize)
(remove): Specify bad argument behaviours.
* lib/std/package.lua (find, insert, mappath, normalize)
(remove): Use argscheck to diagnose bad arguments.
Add usage examples to LDocs.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>