Implement rust_aconfig_library

rust_aconfig_library generates src/lib.rs and uses it to build library variants (dylib, rlib-rlib_std, and rlib-dylib_std) as of what `rust_library` produces

Test: go test
Change-Id: I6c4603691d4306c463c2e9521f5c11c30765b1e3
diff --git a/aconfig/aconfig_declarations.go b/aconfig/aconfig_declarations.go
index 565d185..d1d1578 100644
--- a/aconfig/aconfig_declarations.go
+++ b/aconfig/aconfig_declarations.go
@@ -17,8 +17,9 @@
 import (
 	"android/soong/android"
 	"fmt"
-	"github.com/google/blueprint"
 	"strings"
+
+	"github.com/google/blueprint"
 )
 
 type DeclarationsModule struct {