Do not pass BuildContext to GeneratorTask
We have previously decomposed OutputStream creation and refresh
notification. Now move the refresh notification processing to
YangToSourcesProcessor.
This improves the plugin in that:
- WriteTask is simpler (and smaller in some runtimes)
- BuildContext interactions are more localized
- BuildContext executes on the initial thread, improving its
confinement
JIRA: YANGTOOLS-745
Change-Id: Ia192dbe5134349a78f2290bbf7a9b7ff9d236746
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>