wire: store value expressions in package variables (google/go-cloud#135)

The code assigns a local unnecessarily, but this should have no
appreciable effect on semantics, just readability.

Fixes google/go-cloud#104

internal/wire/testdata/VarValue/foo/foo.go

@@ -17,10 +17,10 @@ package main
 import "fmt"
 
 func main() {
-	// Value should be deferred until function call.
+	// Mutating value; value should have been stored at package initialization.
 	msg = "Hello, World!"
 
 	fmt.Println(injectedMessage())
 }
 
-var msg string
+var msg string = "Package init"

internal/wire/testdata/VarValue/out.txt

@@ -1 +1 @@
-Hello, World!
+Package init