decl_module! print better error on duplicate reserved keyword (#6384)

* `decl_module!` print better error on duplicate reserved keyword

This prints a better error message on duplicated reserved keywords,
instead of complaining because of missing `origin`.

* Review feedback

substrate/frame/support/test/tests/decl_module_ui/reserved_keyword_two_times_integrity_test.rs

@@ -0,0 +1,7 @@
+frame_support::decl_module! {
+	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+		fn integrity_test() {}
+
+		fn integrity_test() {}
+	}
+}

substrate/frame/support/test/tests/decl_module_ui/reserved_keyword_two_times_integrity_test.stderr

@@ -0,0 +1,25 @@
+error: `integrity_test` can only be passed once as input.
+ --> $DIR/reserved_keyword_two_times_integrity_test.rs:1:1
+  |
+1 | / frame_support::decl_module! {
+2 | |     pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+3 | |         fn integrity_test() {}
+4 | |
+5 | |         fn integrity_test() {}
+6 | |     }
+7 | | }
+  | |_^
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0601]: `main` function not found in crate `$CRATE`
+ --> $DIR/reserved_keyword_two_times_integrity_test.rs:1:1
+  |
+1 | / frame_support::decl_module! {
+2 | |     pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+3 | |         fn integrity_test() {}
+4 | |
+5 | |         fn integrity_test() {}
+6 | |     }
+7 | | }
+  | |_^ consider adding a `main` function to `$DIR/tests/decl_module_ui/reserved_keyword_two_times_integrity_test.rs`

substrate/frame/support/test/tests/decl_module_ui/reserved_keyword_two_times_on_initialize.rs

@@ -0,0 +1,11 @@
+frame_support::decl_module! {
+	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+		fn on_initialize() -> Weight {
+			0
+		}
+
+		fn on_initialize() -> Weight {
+			0
+		}
+	}
+}

substrate/frame/support/test/tests/decl_module_ui/reserved_keyword_two_times_on_initialize.stderr

@@ -0,0 +1,25 @@
+error: `on_initialize` can only be passed once as input.
+  --> $DIR/reserved_keyword_two_times_on_initialize.rs:1:1
+   |
+1  | / frame_support::decl_module! {
+2  | |     pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+3  | |         fn on_initialize() -> Weight {
+4  | |             0
+...  |
+10 | |     }
+11 | | }
+   | |_^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0601]: `main` function not found in crate `$CRATE`
+  --> $DIR/reserved_keyword_two_times_on_initialize.rs:1:1
+   |
+1  | / frame_support::decl_module! {
+2  | |     pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+3  | |         fn on_initialize() -> Weight {
+4  | |             0
+...  |
+10 | |     }
+11 | | }
+   | |_^ consider adding a `main` function to `$DIR/tests/decl_module_ui/reserved_keyword_two_times_on_initialize.rs`