below.

Warning: This commit breaks a few things but should work as expected for
simpler use cases. For efforts to use theming while simultaneously using
the default file system loader that wraps Pongo2's built-in loader, this
will likely no longer function as expected.

What we need to do is refactor a substantial portion of this code such
that we can have greater control over where files are being loaded from.
This appears to be potentially easier than I expected when I wrote the
TODO (modifying the Get() behavior in PongoVFSLoader is probably all we
need to do), but I'd like to remove any reliance on the default loader
and force it to use our VFS implementation.

The advantage to this is that we can remove some code and simplify a few
use cases. This means it wouldn't be necessary in default cases to use
the VFS layer, or sensible defaults could be opted for instead, which
would obviate most of the VFS configuration entirely. Then, for more
interesting or customized use cases, we could expose an easier-to-use
API that allows users to attach VFS instances directly instead of
wrapping them via VFS' integration collection (thereby no longer
requiring users to build the collection themselves).