bundle2: remove _getbundleextrapart

Extensions can now add new part generators with the getbundle2partsgenerator
decorator instead.

--HG--
extra : rebase_source : 5b5b6b06f6ee39bb390b194ffa02b2f64a279765
bundle2: client side support for a part to import external bundles

Bundle2 opens doors to advanced features allowing to reduce load on
mercurial servers, and improve clone experience for users on unstable or
slow networks.

For instance, it could be possible to pre-generate a bundle of a
repository, and give a pointer to it to clients cloning the repository,
followed by another changegroup with the remainder. For significantly
big repositories, this could come as several base bundles with e.g. 10k
changesets, which, combined with checkpoints (not part of this change),
would prevent users with flaky networks from starting over any time
their connection fails.

While the server-side support for those features doesn't exist yet, it
is preferable to have client-side support for this early-on, allowing
experiments on servers only requiring a vanilla client with bundle2
enabled.

--HG--
extra : rebase_source : 04548c1a4a17894b1f5cb0b209d6abf6c9c54a7e
extra : amend_source : f430ee28dcab0c1d441dbb40c890ea266c5e443b