[BUGFIX] Ignore missing folders from psr-4 definitions

Even when generating an optimized class map for psr-4
classes, Composer gracefully ignores folders defined
in psr-4 autoload section, if they do not exist.

TYPO3 however fails with an exception.

Also change the logic to always generate a class map
for improved class loading performance. This happened
anyway already when calling the autoload generation twice.

Resolves: #82542
Releases: master, 8.7
Change-Id: I4c3ab8ff7affeba25313d93e40e35f78bb78492d
Reviewed-on: https://review.typo3.org/54207


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>