Add phpstan-no-private to catch calls of seemingly private functions

[westonruter]

Feature Description

As suggested by @swissspidy in #730 (comment):

I can also highly recommend https://packagist.org/packages/swissspidy/phpstan-no-private to catch calls of seemingly private functions.

Nevertheless, I noted:

I love this package, but I wonder if this is the best for Performance Lab considering that the code in question will end up in core and thus it is expected that many calls to internal/private functions will be made? As it stands right now...

And in response, @swissspidy:

Good point! 😃

I think it can't hurt to use it anyway, since not everything will land in core. Places where usage is OK can be ignored in the PHPStan config accordingly. But maybe in another PR :-)

All the Perflab_SQLite_ reports are actually false positives! I'll get this fixed in a new release. (Edit: see swissspidy/phpstan-no-private#2)
They are not properly documented though. A private function ...() method does not need @access private annotations. That's against the coding standards.

[swissspidy]

Especially in OD there are lots of functions/methods/classes marked as private but used in other places of the plugin. This currently triggers an error, but that doesn't seem right. I'll need to investigate that first, see swissspidy/phpstan-no-private#23

Legit errors I spotted at first glance:

• _wp_image_meta_replace_original usage in plugins/webp-uploads/hooks.php
• WP_HTML_Text_Replacement usage in plugins/optimization-detective/class-od-html-tag-processor.php

[westonruter]

• WP_HTML_Text_Replacement usage in plugins/optimization-detective/class-od-html-tag-processor.php

This is an intentional private usage.