support sbt plugin snapshots, improve sonatype http tests

[a1kemist]

Purpose

• Generate valid maven-metadata.xml for sbt plugin snapshots
• Improve sonatype integration testing (http)
• Add http scripted tests for snapshot, sbt-plugin, and sbt-plugin snapshot cases

Background

When publishing an sbt plugin snapshot to sonatype, the publish is rejected due to an invalid maven-metadata.xml. It looks like most of the support for publishing sbt plugins was added in #65, but sbt plugin snapshots end up in a similar situation.

This PR follows the lead taken in #65 by adding the minimum required java sources in order to override the snapshot metadata generation for sbt plugins.

[hamnis]

First of all, thanks for contributing this! Its highly appreciated. Sorry for the long delay in reviewing.

I'm not really a fan of including the maven code verbatim here.

I get that this is final and package private in Maven, but we should use maven, not reimplement it. Another approach could be to put things in the same original maven package and make those use the internal classes then expose only the needed changes. If this is possible of course.

Since I have already accepted #65 I can't reject this on this alone, maybe a follow up PR is a better approach. WDYT ?

[soceanainn]

@hamnis any chance that this could be merged? It would be great to finally get full support for SBT plugins, even if it means having to follow up with another PR later to clean up the approach taken with the Maven java code

[hamnis]

@soceanainn yes, i will merge this, but we will need a cleanup of this as a new PR.