Support generated source

[Atry]

At the moment this plugin set the source file URL to Github. Unfortunately, this approach creates dead link to generated source file.

There should be a new plugin that set source file URL to Sonatype page, e.g. https://oss.sonatype.org/service/local/repositories/public/archive/org/typelevel/cats-core_sjs0.6_2.12/1.0.0-MF/cats-core_sjs0.6_2.12-1.0.0-MF-sources.jar/!/cats/CartesianArityFunctions.scala

Note that the new plugin should be enabled only when isSnapshot is true.