Fix many minor code style issues

[thiemowmde]

I found that all these changes are to minor for individual patches. They also interfere with each other. If you disagree with a change but want to merge the rest, please tell me, and I will split it off.

• Make protected stuff private by default. We already did this almost everywhere.
• Remove since tags from private stuff.
• Make @param tags as specific as possible.
• Avoid full qualified class names.
• Remove a not needed @var tag.
• Fix my name.
• Update license tags according to SPDX version 3.
• Start using the PHPUnit's forward-compatible TestCase alias.
• Drop the custom PHPCS rule set, and use the upstream Wikibase rule set instead.

[JeroenDeDauw]

this is a breaking change, cannot be done before next breaking rel, and might not make sense in there

Wikibase is not the only user of this lib btw

[JeroenDeDauw]

-2 as per inline comment

[thiemowmde]

Do you prefer this to be split in a separate patch, or not done at all?

[JeroenDeDauw]

The cost benefit of doing this seems very dubious. You need a breaking rel which is certainly not justified by just that change and need to verify such a change is fine for all users.

[thiemowmde]

I reverted all the protected/private and @since stuff for now.

[JeroenDeDauw]

+0

[mariushoch]

Needs rebase.

[thiemowmde]

Rebased and updated (see above).