A human-friendly DSL for ATProto Lexicons

Introduce --flat option to MLF generation master

`mlf generate lexicon` has a `--flat` option to generate a lexicon file without any nesting. So intead of generating `com/example/forum/thread.json`, it generates `com.example.forum.thread.json`. This commit adds the same `--flat` option to generate MLF files via `mlf generate mlf`, hence generating `com.example.forum.thread.mlf` instead of `com.example.forum.thread.mlf`.


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