[Compiler] Implement module loading.

Summary:
This branch implements an abstract class AbstractModuleLoader with
factory registration. On top of it, it implements a
JavaScriptModuleLoader, that loads qml modules implemented in js.
It provides a static method ModuleLoading::loadModule that will look
for an appropriate loader and will (asynchronously) load the module.

This branch removes symbol table, which turned out, not to make any
sense.

Reviewers: jangmarker

Reviewed By: jangmarker

Projects: QmlWeb

Maniphest Tasks: T493

Differential Revision: https://phabricator.kde.org/D335