feat(developer): add package validation

Adds a package-validation.ts module, plus adds a call to the validation
step in the compilers.

Rearranges the kmc-package files to match other compilers better.

Removes `default` from exports.

And finally, adds ERROR_PackageCannotContainBothModelsAndKeyboards and
WARN_PackageShouldNotRepeatLanguages messages, plus tests.