The path information was an optional "filename" component of crate
directive AST. It is now replaced by an attribute with metadata named
"path".
With this commit, a directive
mod foo = "foo.rs";
should be written as:
#[path = "foo.rs"]
mod foo;
Closes issue #906.
Under this scheme when parsing foo.rc the parser will also look for
foo.rs to fill in the crate-level module, and when evaluating a
directory module directive it will look for a .rs file with the
same name as the directory.