Remove special case for sdk_version: "none"

The dependency handling for sdk_version: "none" (which propagated to
!sdkDep.hasStandardLibs()) was very similar to the normal useModule
case.  Combine the cases by making decodeSdkDep set useModule: true
and put the system modules in modules for the sdk_version: "none"
case.

Test: TestClasspath
Change-Id: Icc9ff4d43a38da25cc0e3628be95951d61773ad5
diff --git a/java/sdk.go b/java/sdk.go
index c6a9a73..d8584d2 100644
--- a/java/sdk.go
+++ b/java/sdk.go
@@ -192,13 +192,16 @@
 			ctx.PropertyErrorf("sdk_version",
 				`system_modules is required to be set to a non-empty value when sdk_version is "none", did you mean sdk_version: "core_platform"?`)
 		} else if systemModules == "none" {
-			// Normalize no system modules to an empty string.
-			systemModules = ""
+			return sdkDep{
+				noStandardLibs: true,
+			}
 		}
 
 		return sdkDep{
+			useModule:      true,
 			noStandardLibs: true,
 			systemModules:  systemModules,
+			modules:        []string{systemModules},
 		}
 	case "core_platform":
 		return sdkDep{