Missing `mill.runner.MillBuild` external module

[lefou]

Looks like we lost mill.runner.MillBuild external module, which allowed us to get the meta build level count.

• introduced at: https://github.com/com-lihaoyi/mill/pull/2719
• removed at: https://github.com/com-lihaoyi/mill/pull/3717

Usage:

> MILL_VERSION=0.12.0 mill show mill.runner.MillBuild/levelCount
2

> MILL_VERSION=0.12.14 mill show mill.runner.MillBuild/levelCount
2

> MILL_VERSION=1.0.0-RC1 mill show mill.runner.MillBuild/levelCount
[1/1] show
[1/1, 1 failed] ============================== show mill.runner.MillBuild/levelCount ============================== 5s
1 tasks failed
show Cannot resolve external module mill.runner.MillBuild

[lefou]

> MILL_VERSION=0.12.14 mill inspect mill.runner.MillBuild/levelCount
[build.sc-61/65] compile
[build.sc-61] [info] compiling 1 Scala source to /tmp/mill-2/out/mill-build/compile.dest/classes ...
[build.sc-61] [info] done compiling
[1/1] inspect
mill.runner.MillBuild.levelCount(MillBuild.scala:13)
    Count of the nested build-levels, the main project and all its nested meta-builds.
    If you run this on a meta-build, the non-meta-builds are not included.

Inputs:

[1/1] ============================== inspect mill.runner.MillBuild/levelCount ============================== 1s

> MILL_VERSION=0.12.14 mill inspect mill.runner.MillBuild/
[1/1] inspect
mill.runner.MillBuild(BaseModule.scala:19)

Inherited Modules:
    mill.define.ExternalModule
    mill.runner.MillBuild

[1/1] ============================== inspect mill.runner.MillBuild/ ==============================