Adopt builder plugin resolver API from RequireJS

[kitsonk]

While this isn't strictly a loader thing, this seems to be the right place to discuss this at the moment. In Dojo 1, build resolvers for plugins were part of the builder.

RequireJS took a different approach and extended the plugin API to accommodate build time resolution of the plugins:

• write(pluginName, moduleName, write) - called by the builder when the plugin would output something that should belong in an built layer.
• onLayerEnd(write, data) - called by the builder when the end of a layer is reached, either for the plugin to add something or reset some sort of state.
• writeFile(pluginName, name, parentRequire, write) - called by the builder when the plugin needs to write out an alternative file.
• pluginBuilder - an alternative MID to be used by the builder, therefore the build logic doesn't have to be included in the normal plugin.

I would recommend that we adopt this API and focus our build toolchain on leveraging it.

[kitsonk]

At this point, we do not anticipate needing to adopt this API to provide optimised builds.