lexical inventory retrieval only works with external morph retrieval

[balmas]

the lexical inventory lookup is currently too tied to the lookup of morphological data from external services.

for a viewer mode, it should be possible to have morph retrieval offline (i.e. where morph data is retrieved only from the document) and still lookup forms in a lexical inventory (this time by lemma rather than uri).

Needed for @jmharrington's github workflow.