loader: Update to accept file:// URLs.

The file loader splits paths on the first colon character and uses the
left-hand side for the prefix to root the document at under data. On
windows this is problematic because of drive lettesr (e.g., C:\X\Y\Z
is interpreted as load file at \X\Y\Z under data.C.

This change updates the loader to accept file:// URLs. This way
callers can unambiguously specify filenames that contain colon
characters. For now this will mainly be used by VS Code and other
programmatic callers. In future we can support other schemes (e.g., http).

Fixes #1505

Signed-off-by: Torin Sandall <torinsandall@gmail.com>