unibuilds need to be rebuilt if their plugins are

Basically, this branch is moving back to the pre-0.9.0 model where you
tell if a dependent package has changed by explicitly putting its
relevant files in your watchset, instead of by including a "build
identifier" which requires a relatively complex calculation.

Notably, let's say you had package P with a plugin, and a package Q that
depends on it.  If a file in P that's neither part of the plugin nor the
package.js changes, you don't have to rebuild Q... but it did change the
0.9.0 "build identifier", which caused an unnecessary rebuild!

tools/isopack-compiler.js

@@ -222,6 +222,11 @@ var compileUnibuild = function (options) {
       if (unibuild.pkg.name === isopk.name)
         return;
       pluginProviderPackageNames[unibuild.pkg.name] = true;
+      // If other package is built from source, then we need to rebuild this
+      // package if any file in the other package that could define a plugin
+      // changes.
+      watchSet.merge(unibuild.pkg.pluginWatchSet);
+
       if (_.isEmpty(unibuild.pkg.plugins))
         return;
       activePluginPackages.push(unibuild.pkg);