AutoloadGenerator: Filter PSR4-compliant classes instead of ignoring directories

AutoloadGenerator: Filter PSR4-compliant classes instead of ignoring directories

Per discussion in T166010, we're going to handle class aliases (e.g. for
BC) by including the class_alias() call in the same file as the target
class. When the target class is a PSR4-compliant class, we still need to
pick up that alias for inclusion in autoload.php.

Thus, instead of excluding whole directories, we need to process the
files and filter out only those found classes that are PSR4 compliant.

Bug: T204983
Change-Id: I1c516998df368531c90ea54acc5be8be96e1db6c