[BUGFIX] Allow array of paths in psr-4 autoload definition

When defining a namespace prefix for psr-4 autoload in composer.json,
it is possible to specify multiple paths as array.

Thus, we must handle the array case properly and not falsely assume
that there is only a path string.

Resolves: #77365
Releases: 7.6, master
Change-Id: If96f4ce9eb3f4466e8cc620025b1848681b56011
Reviewed-on: https://review.typo3.org/49331


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Helmut Hummel <typo3@helhum.io>