Prefer caching the relative path to a class in the ClassLoader

[LukeTowers]

Adds Filesystem->isAbsolutePath() and changes the ClassLoader->autoloadPackage() logic to prefer storing a relative path to a class file over an absolute one.

This solves an issue that occurs when deploying using an atomic deployment strategy on a server where the storage folder is shared between deployments.

[what-the-diff]

• Added a new method to the Filesystem class that determines if a path is absolute
• Updated ClassLoader::resolvePath() so it uses this new method instead of its own implementation
• Updated ClassLoaderTest::testResolvePath() with some additional test cases for resolving paths relative to the base path and also added tests for autoloading packages using an absolute or relative path (previously only supported loading from within the base directory)