Plugin code is often loaded into the workspace in Eclipse, and is
therefore part of the default project path. Detect this at startup
time by looking for gerrit.buck-out system property and building a
new classpath consisting only of the primary classes and JARs.
The GWT compiler, tests and plugins are excluded from the classpath,
preventing weird collisions with the dynamically loaded plugins from
$site_path/plugins.
PluginLoader recognizes the split output directory used by Eclipse and
loads the latest classes files ahead of anything from the JAR in
$site_path/plugins. This makes development of most code changes quick
by hiding the staleness of the installed JAR.
Change-Id: I4b879eebbcb332384c4e747d3f0c4b5c948c5fed
130afbd65e