Replace old dependency compilation with new pkg_graph approach

This removes the old dependency-specific compilation logic in favour of
flattening the graph into a list of compilation steps and using the same
`compile_pkg` function to compile all packages, including at the
top-level package.

forc/src/ops/forc_abi_json.rs

@@ -11,8 +11,8 @@ pub fn build(command: JsonAbiCommand) -> Result<Value, String> {
         minify_json_abi: command.minify,
         ..Default::default()
     };
-    let (_bytes, json_abi) = crate::ops::forc_build::build(build_command)?;
-    let json_abi = json!(json_abi);
+    let compiled = crate::ops::forc_build::build(build_command)?;
+    let json_abi = json!(compiled.json_abi);
     if let Some(outfile) = command.json_outfile {
         let file = File::create(outfile).map_err(|e| e.to_string())?;
         let res = if command.minify {