Don't call android.PathForModuleSrc(ctx).String() to get ModuleDir

ctx.ModuleDir is much cheaper than android.PathForModuleSrc(ctx).String().

Test: builds
Change-Id: I81819088d9564b06a0336a59a45f2b110b0bd9d6
diff --git a/rust/builder.go b/rust/builder.go
index 72e5be1..162d1aa 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -288,7 +288,7 @@
 	}
 
 	// Disallow experimental features
-	modulePath := android.PathForModuleSrc(ctx).String()
+	modulePath := ctx.ModuleDir()
 	if !(android.IsThirdPartyPath(modulePath) || strings.HasPrefix(modulePath, "prebuilts")) {
 		rustcFlags = append(rustcFlags, "-Zallow-features=\"\"")
 	}
@@ -436,7 +436,7 @@
 	docTimestampFile := android.PathForModuleOut(ctx, "rustdoc.timestamp")
 
 	// Silence warnings about renamed lints for third-party crates
-	modulePath := android.PathForModuleSrc(ctx).String()
+	modulePath := ctx.ModuleDir()
 	if android.IsThirdPartyPath(modulePath) {
 		rustdocFlags = append(rustdocFlags, " -A warnings")
 	}