Avoid Rust source provider rule duplication

Until now, source provider modules duplicated the rule to generate the
source for each variant. Add a inter-variant dependency between the
source and the other variants (e.g. rlib, dylib) to avoid this
duplication. Add documentation on this behaviour.

Bug: 162588681
Test: m
Change-Id: I41c9e2220f8875245415e17374852e540dfd47ec
diff --git a/rust/bindgen_test.go b/rust/bindgen_test.go
index 191da9b..2b6c480 100644
--- a/rust/bindgen_test.go
+++ b/rust/bindgen_test.go
@@ -41,7 +41,7 @@
 			export_include_dirs: ["static_include"],
 		}
 	`)
-	libbindgen := ctx.ModuleForTests("libbindgen", "android_arm64_armv8-a").Output("bindings.rs")
+	libbindgen := ctx.ModuleForTests("libbindgen", "android_arm64_armv8-a_source").Output("bindings.rs")
 	// Ensure that the flags are present and escaped
 	if !strings.Contains(libbindgen.Args["flags"], "'--bindgen-flag.*'") {
 		t.Errorf("missing bindgen flags in rust_bindgen rule: flags %#v", libbindgen.Args["flags"])
@@ -73,7 +73,7 @@
 		}
 	`)
 
-	libbindgen := ctx.ModuleForTests("libbindgen", "android_arm64_armv8-a").Output("bindings.rs")
+	libbindgen := ctx.ModuleForTests("libbindgen", "android_arm64_armv8-a_source").Output("bindings.rs")
 
 	// The rule description should contain the custom binary name rather than bindgen, so checking the description
 	// should be sufficient.