Rewrite autoloader

[luigifab]

Description

This PR fix #1780 and replace #1817 and #2300.
It's created from @woutersamaey, @Asfolny, @Flyingmana, and me ideas.

There is a tiny breaking change:
for example with Ext4mage/Orders2csvpro, there are:
<source_model>ext4mage_orders2csvpro_model_source_creditmemofiles</source_model>
This does not work any more, because ucwords is removed in this PR.

I checked with Blackfire, this is a performance regression (see comment bellow).
But from ~5 ms to ~10 ms.

I didn't tested with #1969.

Contribution checklist

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[tmotyl]

Please add more info and context around this change.

• why?
• what is changed
• Why is it breaking
• Why is it slower?

[fballiano]

this file should get removed by #2411

[luigifab]

Original version is:
return @include str_replace(' ', DIRECTORY_SEPARATOR, ucwords(str_replace('_', ' ', $class))) . '.php';

The simplified version used in this PR (all tests bellow with Blackfire with it):
$path = str_replace(['_', '\\', ' '], DIRECTORY_SEPARATOR, $class) . '.php';

First, with return @include $path;

Second, with stream_resolve_include_path (link)

Third, with foreach get_include_path (link)

The difference between stream_resolve_include_path (12.3 - 10.0 - 8.81 ms) and foreach get_include_path (10.4 - 12.1 - 10.8 ms) seems to be negligible.

[luigifab]

Today I realize that perhaps the replace of the space is not necessary? Not tested.
For the removal of ucwords, perhaps I can keep it, I didn't tested, or I can remove it only on 20.0.

[fballiano]

lib/phpseclib is no longer with us :-)

[fballiano]

this has a conflict now, I'll move it to draft

[Flyingmana]

What speaks against first finishing/merging #2300 to fix the bug, and then do the performance improvement separately?

[luigifab]

Closed for #2300, and also because #1780 is now fixed.