[INTERNAL] lib/jsdoc: clearly separate missing vs. imported symbols
[INTERNAL] lib/jsdoc: clearly separate missing vs. imported symbols

When gaps in the inheritance tree and interface relationships are closed
with synthetic symbols, they either can be external symbols, known from
the api.json files of predecessor libraries ('import') or they can be
missing, e.g. due to a typo in the reference or due to their visibility
(reference to a private symbols). Only the latter case should be
reported as a warning.

Change-Id: Ide0dd44c849b3b0f0c1e3e0f64dd8c3ee99ed15a

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5