Add a function to create config_setting(s)

The use case for this is creating config_setting(s) specific to an apex
variant and selecting stub/impl in that config_setting. We likely need
only a handful of such config_setting(s), but determining that list
requires iterating the build graph.

Test: go test ./bp2build
Change-Id: I9aa552e3d0bcf67513023c3a7d4bbf8fae464ee4

starlark_fmt/format.go

@@ -99,6 +99,16 @@ func PrintStringIntDict(dict map[string]int, indentLevel int) string {
 	return PrintDict(valDict, indentLevel)
 }
 
+// PrintStringStringDict returns a Starlark-compatible string formatted as dictionary with
+// string keys and string values.
+func PrintStringStringDict(dict map[string]string, indentLevel int) string {
+	valDict := make(map[string]string, len(dict))
+	for k, v := range dict {
+		valDict[k] = fmt.Sprintf(`"%s"`, v)
+	}
+	return PrintDict(valDict, indentLevel)
+}
+
 // PrintDict returns a starlark-compatible string containing a dictionary with string keys and
 // values printed with no additional formatting.
 func PrintDict(dict map[string]string, indentLevel int) string {