Validate absolute path for substitute_type (#577)

* Remove commented code

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Ensure substitue path is global absolute

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Absolute path for build.rs

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Add ui test that fails to compile

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

testing/ui-tests/src/incorrect/substitute_path_not_absolute.rs

@@ -0,0 +1,7 @@
+#[subxt::subxt(runtime_metadata_path = "../../../artifacts/polkadot_metadata.scale")]
+pub mod node_runtime {
+    #[subxt::subxt(substitute_type = "sp_arithmetic::per_things::Perbill")]
+    use sp_runtime::Perbill;
+}
+
+fn main() {}

testing/ui-tests/src/incorrect/substitute_path_not_absolute.stderr

@@ -0,0 +1,5 @@
+error: The substitute path must be a global absolute path; try prefixing with `::` or `crate`
+ --> src/incorrect/substitute_path_not_absolute.rs:4:9
+  |
+4 |     use sp_runtime::Perbill;
+  |         ^^^^^^^^^^