peer: add a `path` attribute to peer It will start being set in the coming changesets.

peer: have a common constructor and use it For now it does not do much, but we will extend it to also store a path object soon.

peer: use a dedicated name for the `peer` constructor We want to change the argument it takes, so we rather make them different function.

peer: dissolve `_peerlookup` into its last two callers This is about to need more changes and the function won't be useful. We do it early to clarify later changes.