[Enhancement]: Create variants for Java and others

[merlijn-sebrechts]

What changes would you like?

We'd like to have variants for Java, CDT, and other types. Some people have worked on this, but we haven't had a finished PR yet.

• This is the latest attempt: https://github.com/snapcrafters/eclipse/pull/56
  • It's a good PR, with the limitation that it uses strict confinement.
• https://github.com/snapcrafters/eclipse/pull/10
  • This is another unfinished try.