[validation required] scala_library without sources does not reflect main_class in generated Manifest

Will reproduce this tomorrow but wanted to report that it seems like scala_library withotu soruces packager ignores `main_class` attribute and doesn't generate correct Manifest file.

In some cases we do need jar without sources and mention main class that comes from runtime dep.

Start by reproducing the reported case with a scala_library that has no sources but sets main_class and relies on a runtime dependency. Inspect the generated JAR Manifest and compare it with a source-bearing scala_library; done means the no-source package reflects the declared main class correctly.

Written by the indexing model from the issue text.