Improve "Getting packages..." message for Bower and Pub 

@dinhviethoa

With #2675, the fixing aspect of this change becomes less urgent, but possibly still relevant for future, when multiple jobs could be able to run simultaneously.

The improving aspect is completely relevant, though: for Bower, the message will now look like "Getting Bower packages... 37 of 48", showing the real downloading progress. For Pub, this is harder to achieve, because Tavern hides the total work and current progress from the client.
The inherited `Job.run()` already takes `ProgressMonitor` as a parameter, so, unlike other instance variables in `PackageManagementJob`, it's a transient thing presumably valid only for the duration of a `run` call. I wouldn't want to mix permanent object properties with such cached references.