Use Path.join() to create paths instead of Url.path() (#781)

We can't assume that the path section of a file:// URL is a valid
operating system path, so instead we join the root and the source using
Rust's std::path API. This still handles absolute paths the same.

This commit resolves issues where we couldn't use ethcontract-derive
on Windows (e.g. #729)

Co-authored-by: videogame hacker <[email protected]>

Author: Jaxkr

ethcontract-generate/src/source.rs

@@ -110,7 +110,7 @@ impl Source {
         let url = base.join(source.as_ref())?;
 
         match url.scheme() {
-            "file" => Ok(Source::local(url.path())),
+            "file" => Ok(Source::local(root.join(source))),
             #[cfg(feature = "http")]
             "http" | "https" => match url.host_str() {
                 Some("etherscan.io") => Source::etherscan(