wiggle: witx paths use shell expansion

instead of always being relative to CARGO_MANIFEST_DIR, each use site is
responsible for either putting that variable or another one (set by a
build.rs) at the start of witx paths.

crates/wiggle/macro/src/lib.rs

@@ -90,10 +90,7 @@ use syn::parse_macro_input;
 /// ```
 #[proc_macro]
 pub fn from_witx(args: TokenStream) -> TokenStream {
-    let mut config = parse_macro_input!(args as wiggle_generate::Config);
-    config.witx.make_paths_relative_to(
-        std::env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR env var"),
-    );
+    let config = parse_macro_input!(args as wiggle_generate::Config);
 
     let doc = config.load_document();
     let names = wiggle_generate::Names::new(&config.ctx.name, quote!(wiggle));