Bzlmod: maven.install repositories has silent last-wins semantics

[plobsing]

When the main module invokes maven.install more than once, the last invocation's value for repositories is what gets used. This includes the implicit repositories = [DEFAULT_REPOSITORIES] when the last call does not specify an explicit value for the argument.

This seems like an API design question. Maybe we do want to support multiple invocations and apply some kind of merge semantics to the repositories argument. Or maybe we want to support multiple invocations, but only one can have a repositories argument and that's what gets used (and there should be an error if there's more than one). Or maybe we don't support more than one invocation, in which case calling maven.install multiple times should be an error. I don't think the current behaviour of letting the user call maven.install multiple times, but silently ignoring some of their explicit input is a desirable behaviour.