Compile / PB.targets seems to be required to generate Test scoped protos

[a1kemist]

First off I would just like to say thank you to all the contributors of scalapb for the fine work that has been done on this project. 🙇

While working on some integration tests, I needed to create some protos that were only going to be used for testing. To my surprise, it appears that Compile / PB.targets needs to be defined in order for Test / PB.targets to evaluate. (The sources under test have no protos themselves)

I have created a branch that adds two examples under examples/* to reproduce the issue (examples/testOnly and examples/testPlusCompile).

The testPlusCompile example build.sbt looks like the following and builds correctly:

scalaVersion := "2.13.8"

Compile / PB.targets := Seq(
  scalapb.gen() -> (Compile / sourceManaged).value / "scalapb"
)

Test / PB.targets := Seq(
  scalapb.gen() -> (Test / sourceManaged).value / "scalapb"
)

I would expect that a user would be able to do the following in order to generate protos that are only used for testing:

scalaVersion := "2.13.8"

Test / PB.targets := Seq(
  scalapb.gen() -> (Test / sourceManaged).value / "scalapb"
)

Please let me know if I can provide any additional information and I would be happy to help.