* libs-scala/ports: Wrap socket ports in a type, `Port`.
* sandbox: Use `Port` for the API server port, and propagate.
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* extractor: Use `Port` for the server port.
* ports: Make Port a compile-time class only.
* ports: Allow port 0; it can be specified by a user.
* ports: Publish to Maven Central.
* Avoid opening a server to the world when finding a free port.
This is very annoying on macOS because we get a focus-stealing popup for
a split second, asking for permission to allow the server through the
firewall. The popup pretty much always disappears before it can even be
read, when the server is closed.
This is almost certainly not an attack vector, because:
- we only do this in tests,
- the server is open for only a few milliseconds,
- nothing is served,
- and finding the port is tricky, because it's effectively random.
Nevertheless, it's very annoying.
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* Extract a Bazel package for finding free ports.
We seem to do it in 4 different places, which I think is enough to
remove the duplication.
