Workaround mapped value before task completed error for excluded task

[ejona86]

When using provider.map, Gradle has checks that can fail if the provider
is used too early. For example:

Querying the mapped value of provider(interface java.util.Set) before
task ':p1:compileScala' has completed is not supported

However, it seems those checks can also fire even if the incomplete task
is a properly registered dependency and only when accessed from an
action. This is possible when using -x to exclude a task. This deserves
a Gradle bug, but let's workaround it in the meantime.

Fixes #550

CC @rougsig, @Thrillpool

This will workaround whatever problem @Toldry experienced, but I expect that was a different issue compared to that seen by Thrillpool. There's a strong risk that there was a legitimate issue in Toldry's build that now will be silent. But without a reproduction not much we can do for something this nuanced.

[rougsig]

I think we should not depend on compile and processResources tasks. We should depend only on own tasks.

We are currently requesting an artifact with the following attributes: LibraryElement=resources, UsageAttribute=javaRuntime. An artifact with that attributes will be created by the processResources task. We can create our own unique attributes to request an artifact from project dependencies. Artifact will be built by own task. Leave as is for non-project dependencies.

Actually is not a solution for the bug, it's a bit change for plugin logic.

[ejona86]

We are currently requesting an artifact with the following attributes

I'm fuzzy on how that is plumbed, but I agree. Right now if you try to add directories generated by GenerateProtoTask to SourceSet.java then you get an infinite loop.

[ejona86]

gradle/gradle#21179 is bug I filed with Gradle. We'll see where that leads.

[rougsig]

See details about compile task here #578