Added --import command-line option

[lefou]

Motivation: Some plugins provide "External Modules" which can be used by mill without modifying the build.sc.

With this PR applied, you can run targets from external modules from the command-line without touching the build.sc.

Example:

$ dev-mill --plugin ivy:de.tototec::de.tobiasroeser.mill.vcs.version_mill0.10.0-M2:0.1.2 show de.tobiasroeser.mill.vcs.version.VcsVersion/vcsState
[1/1] show 
[1/1] show > [1/1] de.tobiasroeser.mill.vcs.version.VcsVersion.vcsState 
{
    "currentRevision": "5f6538e3cb7191c0a8c778f9d47601967b744036",
    "lastTag": [
        "0.10.0-M3"
    ],
    "commitsSinceLastTag": 36,
    "dirtyHash": [

    ]
}

This PR also add support for empty versions in $ivy-imports, which will get expanded to the exact mill version. This makes consuming mill contrib plugins even more easy.

Example: Generating Bloop Config without modyfing the build.sc

mill --plugin ivy:com.lihaoyi::mill-contrib-bloop: mill.contrib.bloop.Bloop/install

[lefou]

@lihaoyi I'm asking for a review or comment here. I'm myself a bit undecided. I think the --plugin options is useful, but I'm not sure about its name, the syntax (especially the "ivy:" prefix to match meaning of $ivy so we can later support others too) and also the new feature of leaving out the version (affects also import $ivy) to substitute the current mill version. The motivation for the version was twofold: convenience in build.sc but also the fact it's hard to escape the variable substitution of the used shell to provide a $MILL_VERSION in the command-line.

[lihaoyi]

@lefou I think it looks reasonable to me; I don't have a use for it, but I imagine others might. Maybe call it --import ivy:foo, for symmetry with the import $ivy.foo syntax we use in the build.sc file?

[lefou]

@lihaoyi Thank you! I like --import.

Potential users are @scala-steward and Metals, which want to run mill with additional external modules.