Fix #134 for publishing with mill 0.12

[carlosedp]

Fixes issue:

❯ ./mill io.kipp.mill.ci.release.ReleaseModule/publishAll
[68/68] ============================== io.kipp.mill.ci.release.ReleaseModule/publishAll ==============================
java.util.NoSuchElementException: head of empty list
    scala.collection.immutable.Nil$.head(List.scala:663)
    scala.collection.immutable.Nil$.head(List.scala:662)
    mill.resolve.Resolve$.$anonfun$instantiateCommand$1(Resolve.scala:151)

This is related to how the Discover method finds the commands in the plugin.

[carlosedp]

@nightscape @ckipp01 I've managed to fix the plugin with this PR.

[nightscape]

@carlosedp I tried another approach where I reversed the traits and objects in the source-specific sub-folders:
https://github.com/ckipp01/mill-ci-release/pull/140/files
If I understood Li Haoyi's comment in Discord correctly, the issue is that the mill.define.Discover needs to be in an object.
How did you test your branch? Did you publishLocal a version of the plugin and then try it on some other project?

[carlosedp]

Gonna take a loot at it!
Yes, you __.publishLocal and use the generated snapshot version in a test project.

[carlosedp]

Closed in favor of #140 which has less code duplication.