R1:99bcf5f112dc

Make `bin/lipsum generate` hanldle generator keys and arguments more clearly

Summary:
Ref T12319. Currently, `bin/lipsum` uses substring matches against human-readable text to chose which objects to generate.

Instead:

- Use separate selector keys which are guaranteed to be unique.
- When a match is exact, select only that generator.
- When a match is ambiguous, fail and warn the user.

Test Plan: Generated several types of objects, tried to generate ambiguous objects like "e".

Reviewers: chad…