ReleasePlugin should be disabled when JvmPlugin is disabled

[Atry]

Because ReleasePlugin requires settings like managedClasspath defined in JvmPlugin

[Atry]

Ping @jroper

[jroper]

I think it would be better to make those dependencies optional, ie depend on managedClasspath only if it's set. I don't think release plugin should depend on JvmPlugin, it's conceivable that you may release an sbt project that doesn't use the JVM.

[Atry]

In order to make these dependencies optional and still keep the same behavior of the current sbt-release, we need the following steps:

1. Create a ReleaseCorePlugin, which does not depend on JvmPlugin.
2. Let ReleasePlugin depend on both ReleaseCorePlugin and JvmPlugin.
3. Move those settings that does not require managedClasspath from ReleasePlugin to ReleaseCorePlugin.

[Atry]

Hi, @jroper ,

Since this PR is a bug fix, could you merge this PR for now?

Without this PR, if some one added sbt-release dependency in /project/plugins.sbt for a multi-project build and disabled JvmPlugin in /non-jvm-subproject/build.sbt for one of these projects, the build would be broken.

[jroper]

I'll merge if you update it to make the dependency on managedClasspath optional. You don't need to do the multiple plugins that you suggested, just use the ? operator, eg:

val maybeClasspath: Option[Classpath] = managedClasspath.?.value

Requiring the JvmPlugin to be enabled in order for the ReleasePlugin to work is unnecessary.

Also, I'd like to see a scripted test added to ensure that the bug is fixed and doesn't regress in future.