Work around Groovy compiler bug that can name classes incorrectly

If a source URL is added to a CompilationUnit and that source URL does
not contain any slashes, the resulting ClassNode in the AST will be
incorrectly named. For example, a URL of 'file:foo.groovy' will produce
a ClassNode named 'file:foo'. The expected name is 'foo'.

This commit works around this problem by adding any URL sources with a
file protocol to the compilation unit as File instances. Any URL sources
that do not have a file protocol continue to be added as URL instances.
Such URLs are still prone to this bug should we be dealing with one
that contains no slashes. A fix for the underlying Groovy bug will
address this possibility.

Fixes #594