Add a p2 multitool

[nedtwigg]

In this PR we expose the Solstice P2 resolution engine to Gradle users.

equoIde {
  p2repo 'https://download.eclipse.org/eclipse/updates/4.26/'
  install 'org.eclipse.platform.ide.categoryIU'
  install 'org.eclipse.releng.java.languages.categoryIU'
  filter {
    excludePrefix 'org.apache.felix.gogo'
    excludePrefix 'org.eclipse.equinox.p2'
    exclude 'org.eclipse.equinox.console'
  }
}

Then you can do:

• gradlew equoList to list all the jars which would be installed
• gradlew equoList --install=org.eclipse.some.id to list all the jars which would be installed for the given target
• gradlew equoList --dumpAll=features to list all features in the repositories (ignores filters)
• gradlew equoList --dumpAll=categories to list all categories in the repositories (ignores filters)

Or at least that's the idea. The spec might change a bit as it progresses.

EDIT: the spec changed a bunch, I'm gonna publish this as-is so that I can have it as a tool for working on the next version.