ipn/ipnlocal: refactor resolveExitNodeInPrefsLocked, setExitNodeID and resolveExitNodeIP

Now that resolveExitNodeInPrefsLocked is the only caller of setExitNodeID,
and setExitNodeID is the only caller of resolveExitNodeIP, we can restructure
the code with resolveExitNodeInPrefsLocked now calling both
resolveAutoExitNodeLocked and resolveExitNodeIPLocked directly.

This prepares for factoring out resolveAutoExitNodeLocked and related
auto-exit-node logic into an ipnext extension in a future commit.

While there, we also update exit node by IP lookup to use (*nodeBackend).NodeByAddr
and (*nodeBackend).NodeByID instead of iterating over all peers in the most recent netmap.

Updates tailscale/corp#29969

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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