Make lazy collectiing of Workspace Artifacts in completion

[vrubezhny]

Make Maven Projects to be loaded/cached lazily when collecting the Workspace Artifacts when gathering completions n order to make content assist faster and more responsive

Issue: #444

[mickaelistria]

How is user made aware that the completion results are incomplete because of lazy loading? If there is nothing to warn them, then they may interpret it as a regression.
The fact that some assertions had to change in tests do showcase that this can be perceived as a regression if we don't make it very clear and usable to users that more results are to be provided.

[angelozerr]

At first we have a progress monitor which notifiés the user that local repo is scanning.

While this scanning folder, lemminx maven completion is disabled to provide completion items from lemminx like dependency tag elelent based on xsd

[mickaelistria]

It would be nice if lemminx-maven introduced some dummy item in the completion result to show users that the list is incomplete. An entry such as "More will be provided after local repo is scanned".
However, I imagine the operation is relatively quick, so in practice, users almost often get the content of the local repo in the proposals, don't hey?

[angelozerr]

My scan of my big lcal repo takes arround 15 seconds.

A thing that we could do when completion is triggered while this scan is to provide artifact which are already scanned

[angelozerr]

entry such as "More will be provided after local repo is scanned".

It existed rhis kind of entry but we removed it because completion item is used to apply it and insert some content not to display some i fo