Group targets by name

[natefinch]

I'm using mage in a monorepo project with a small set of core targets (e.g., test, build, package, deploy, etc.) that are consistent across all "sub-projects" in the monorepo. Each sub-project has its own magefile.go and there is a root-level magefile.go that "aggregates" the targets of the sub-projects' magefile.go files with some namespacing. (I say "aggregates" because currently the aggregation is hand-curated.)

My project layout looks like...

my-project/
  sub-project-A/
    magefile.go
  sub-project-B/
    magefile.go
  sub-project-C/
    magefile.go
  magefile.go

Each sub-project's targets are essentially...

$ mage -l
Targets:
  test           Runs unit tests.
  build         Compiles the project.
  package    Packages the project.
  deploy      Deploys the project.

And the (hand-curated) root-level targets are of the form...

$ mage -l
Targets:
  test:all    Runs all projects' unit tests.
  test:a      Runs project A unit tests.
  test:b      Runs project B unit tests.
  test:c      Runs project C unit tests.
  build:all  Compiles all projects.
  build:a    Compiles the A project.
  build:b    Compiles the B project.
  build:c    Compiles the C project.

It would be super helpful if mage's target import mechanism could do this all on a user's behalf by aggregating imported targets of the same name under a namespace (for example).

Originally posted by @jonasrmichel in https://github.com/magefile/mage/issues/108#issuecomment-425213249