Decouple logic from actual autoloader class filesystem location

[austinpray]

It seems that a lot of the logic relies on the autoloader living in the mu-plugins dir. Since this is just a regular composer library we need to ensure that the relevant locations are passed in as params

https://github.com/roots/bedrock-autoloader/blob/02e61a6f7ff78c2d77599843617f88156c4c6274/src/Autoloader.php#L58

https://github.com/roots/bedrock/issues/299#issuecomment-500117865