refactor: use a function to export package apis.

* specs/package_spec.yaml (find, insert, mappath, normalize)
(remove): Update to latest style.
Add "too many argument" behaviour checks.
Simplify and standardise argument error message comparisons.
* lib/std/package.lua (M): Add module name at element 1.
(find, insert, mappath, normalize, remove): Upgrade to
base.export declarations (for overhead free argcheck calls with
_DEBUG = false) and simplify accordingly.
(package): Improve LDocs.

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