compiler/protogen, internal/strs, internal/impl: expose enum Go name derivation

In order to migrate v1 to wrap v2, we need a way to reproduce
the awful enum "names" that v1 used, which was the concatenation of
the proto package with the Go identifier used for the enum.

To support this:
* Move the camel case logic from compiler/protogen to internal/strs
* Add a small stub in internal/impl to expose this functionality

Change-Id: I8ff31daa9ae541e5788dc04d2e89eae1574877e4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/191637
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/legacy_enum.go b/internal/impl/legacy_enum.go
index 279baa9..4ec31df 100644
--- a/internal/impl/legacy_enum.go
+++ b/internal/impl/legacy_enum.go
@@ -11,10 +11,27 @@
 	"sync"
 
 	"google.golang.org/protobuf/internal/filedesc"
+	"google.golang.org/protobuf/internal/strs"
 	"google.golang.org/protobuf/reflect/protoreflect"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 )
 
+// legacyEnumName returns the name of enums used in legacy code.
+// It is neither the protobuf full name nor the qualified Go name,
+// but rather an odd hybrid of both.
+func legacyEnumName(ed pref.EnumDescriptor) string {
+	var protoPkg string
+	enumName := string(ed.FullName())
+	if fd := ed.ParentFile(); fd != nil {
+		protoPkg = string(fd.Package())
+		enumName = strings.TrimPrefix(enumName, protoPkg+".")
+	}
+	if protoPkg == "" {
+		return strs.GoCamelCase(enumName)
+	}
+	return protoPkg + "." + strs.GoCamelCase(enumName)
+}
+
 // legacyWrapEnum wraps v as a protoreflect.Enum,
 // where v must be a int32 kind and not implement the v2 API already.
 func legacyWrapEnum(v reflect.Value) pref.Enum {