Allow invocation of foo.super tasks from the command line (500USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

See https://github.com/orgs/com-lihaoyi/discussions/6 for other bounties and the terms and conditions that bounties operate under

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We already give them unique selectors and paths on disk, should be possible to make them callable as well. This could be useful for debugging

We should be able to say ./mill foo.bar.super if there is only one overridden task, or if there are multiple overridden tasks then ./mill foo.bar.super.qux.Baz where qux.Baz is the super-suffix computed in mill.define.internal.OverrideMapping.assignOverridenTaskSegments (basically the partially-qualified class name long enough to uniquely define the defining class of the task)

[jseteny]

@lihaoyi I start to work on this