Build: check built files don't contain external dependencies

We just dealt with failures in functional tests that turned out to be caused by `cldr/supplemental` being added as a dependecy in one place, but not properly included in the build in a different place (requirejs build configuration, `exclude` option).

To make it easier to spot this kind of issue, we should write a build task that looks at files built in `dist/` and throws an error if any file contains the Cldrjs library, or any other always-excluded dependency.