gosky: bunch of moves and refactors (#348)
Started with combining `getRepo` and `downloadRepo`, and ended up moving
around a bunch of sub-commands and adding usage notes. individual
commits are descriptive.
did not change any of the behavior/implementation of individual commands
in this PR.
the most disruptive thing is probably moving `createInvites` to `admin
createInvites`.
removed the "rebaseRepo" command entirely.
happy to revert or re-structure, this is just a proposal.
top-level output:
```
COMMANDS:
admin sub-commands for PDS administration
bgs sub-commands for administering a BGS
car sub-commands to work with CAR files on local disk
createFeedGen
debug a set of debugging utilities for atproto
did sub-commands for working with DIDs
getRecord
handle sub-commands for working handles
list print all of the records for a repo or local CAR file
readStream
sync sub-commands for repo sync endpoints
help, h Shows a list of commands or help for one command
```