Seperate the Lassie library from everything else

[hannahhoward]

This is a companion to #256.

Essentially, my thinking is this:

• when we started this project, we said pkg is the Lassie library, but then we realized we needed to put a bunch of other stuff in there
• as a result, it's very not clear which folders under pkg are actually part of using lassie as a library
• I think we need to put everything that goes into Lassie as a library under /pkg/lassie
  • basically if you need it to make a lassie.Lassie, you put it under pkg/lassie
  • my surface read is that covers these folders
    • events
    • session
    • indexerlookup
    • net
    • retriever
    • session
    • types
  • but not
    • aggregateeventrecorder
    • build
    • eventrecorder
    • metrics (which should get deleted see #255)
    • server
    • storage
    • verifiedcar (though this should probably get moved out of repo to go-car)

This isn't a comment on which of these folders would become internal (in pkg/lassie/internal) but we can think of that as well.

[bajtos]

but not

• server

IIUC, this means that Rusty Lassie will need to (re)create the HTTP server. It would be great if there was an example showing how to write such a server. Even better, can Lassie the library export the request handler that people can add into their own servers (or muxers)?