Improve entry point detection

[MaulingMonkey]

I'm just now adding a feature to assume "Main" is the entry point of "Main.java" exists. This is bogus for several reasons - it might not have a main function, despite the name - it might be using a package - there might be multiple ambiguous entry points.

Options:

• Re-introduce cargo_metabuild dependency, read main from package metadata
• Introduce non-metabuild exports for build.rs to invoke (e.g. jerk::build_with_entry_point("Main"))
• Do more parsing automagic
• ???