Add a "classmap-authoritative" configuration setting that can be used to
disable searching the various prefix and fallback directories for
classes that have not been registered with the
Composer\Autoload\ClassLoader class map. This setting can be used to
optimize performance by avoiding a potentially large number of
`file_exists` calls when Composer is being used in a program with
additional autoloader facilities. Use of the setting implies
"optimize-autoloader" to ensure that the most complete class map
possible is generated.
Closes#3603
ad1f8e6c5a