interfaces/builtin: make all interfaces private

[zyga]

The rest of snapd doesn't need to access interface types, all that is
requires is the builtin.Interfaces function, that returns the list of
all known interfaces.

This patch makes interfaces private, adds a helper for testing, that
returns interface by name. Some tests were changed to use s.iface
rather than trying to construct the interface object needlessly. Some
tests were changed to use spec.AddConnectedPlug rather than
iface.MountConnectedPlug so that the interface test code does not need
to know the real interface type.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[chipaca]

Very nice. Thank you!

Also: holy cow. My head hurts now.