Assorted JlinkPlugin improvements

[nigredo-tori]

I finally got around to using JlinkPlugin in my work projects, and there are some things that need work. I'll probably do these myself soon-ish, so this issue is just for documentation.

1. We need more flexible constructors in Ignore. I'm thinking some simple string globbing.
2. Our jlink call should include our classpath in --module-path, since there might be modular JARs there, and these are caught by our jdeps output parser.
3. If we have no explicit javaHome we use java from PATH, which is not necessarily the JVM in which SBT is running. We should use java.home system property instead.

[muuki88]

Thanks for opening an issue for this @nigredo-tori 😍
I'm happy to merge any pull requests you send 😄