Support for javac plugins.

Review Request #3839 — Created May 7, 2016 and submitted — Latest diff uploaded

nhoward_tw, stuhood, zundel

- Introduces a JavacPlugin target.
- Logic to compile plugins and generate their metadata file.
- Logic to compile with in-repo (or external, published) plugins
enabled. Allows two modes:
+ Targets can explicitly depend on a JavacPlugin, and that
plugin will be run when those targets are compiled. The plugin
will itself be compiled first if needed, naturally, so there's
no need to publish it first.
+ "Global" plugins can be specified via a JavacPluginSetup
subsystem. Any plugins configured there will be injected as deps
on every jvm target, and therefore compiled and used in all javac
compilations (except those of the plugin itself).
This allows you to apply "global" plugins (e.g., a linter)
without having to add dependencies everyhwere.
- Ensures that tools.jar is on the classpath when compiling
- Provides a hack to allow tests to pass without needing to set up
the JavacPluginSetup subsystem, since we have hundreds of tests
that use JvmTasks in various ways.
- Removes the truly gratuitous jvm backend dependency from

Note that the plugin API we support was introduced in Java 8, so
we have to temporarily exclude the example plugin from being compiled
in our tests, until we're entirely on Java 8 in our own CIs.

CI passes: