Fix tracking of junctions used in project.conf.

Stream._load() now returns early without resolving build pipeline when
only tracking. Resolving track pipelines does not require to fully
load project configurations when when elements to track are only
junctions.

However build pipelines require to fully load project
configurations. This might not be possible in the case a project
configuration includes a file from a junction that yet needs to be
tracked.

Fixes #565.