Problem Statement/Rationale

Tab completion does not work for collection names with embedded dots.
Initiating tab completion does seem to work if tab is pressed prior to any embedded dots in a collection name (and will complete past dots to the first ambiguous character in a name). However, initiating tab completion with the cursor to the right of the first dot in a collection name produces no effect, instead of completing the remainder of the name.

Steps to Reproduce

With a fresh, empty server running, execute the following in mongosh:
test> db.alpha.bet.insert({})

Pressing tab with the following works correctly:
test> db.alph
Pressing tab with the following does not work (nothing happens):
test> db.alpha.be

Additional Notes

The problem appears to be that the shell is not merging the set of possible collection names with the set of possible function names correctly; after a dot, it only considers function names for completion candidates. The legacy shell does appear to merge these sets successfully so tab completion works well there.