sshpeer: make "instance" a function
The API is that peer modules must provide an "instance" symbol
that is callable to return a peer.
Making "instance" a function instead of an alias to "sshpeer"
makes it easier to monkeypatch the "sshpeer" type. It will also
make it possible to turn instance() into a factory function of
sorts that returns different types based on connection properties.
Differential Revision: https://phab.mercurial-scm.org/D2024
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