fix: panic on error type (#103)

Types in the universe scope (like error) don't have a package.

scenario/universe_type.yml

@@ -0,0 +1,30 @@
+input:
+    input.go: |
+        package universe
+
+        // goverter:converter
+        // goverter:extend Error
+        type Converter interface {
+            Error(source *error) *error
+        }
+        func Error(value error) error {
+            return value
+        }
+success:
+    - generated/generated.go: |
+        // Code generated by github.com/jmattheis/goverter, DO NOT EDIT.
+
+        package generated
+
+        import execution "github.com/jmattheis/goverter/execution"
+
+        type ConverterImpl struct{}
+
+        func (c *ConverterImpl) Error(source *error) *error {
+        	var pError *error
+        	if source != nil {
+        		xerror := execution.Error((*source))
+        		pError = &xerror
+        	}
+        	return pError
+        }

scenario/universe_type_error.yml

@@ -0,0 +1,32 @@
+input:
+    input.go: |
+        package universe
+
+        // goverter:converter
+        type Converter interface {
+            AToB(A) B
+        }
+
+        type A struct {
+            Property int
+        }
+
+        type B struct {
+            Property error
+        }
+error: |-
+    Error while creating converter method:
+        func (github.com/jmattheis/goverter/execution.Converter).AToB(github.com/jmattheis/goverter/execution.A) github.com/jmattheis/goverter/execution.B
+
+    | github.com/jmattheis/goverter/execution.A
+    |
+    |      | int
+    |      |
+    source.Property
+    target.Property
+    |      |
+    |      | error
+    |
+    | github.com/jmattheis/goverter/execution.B
+
+    TypeMismatch: Cannot convert int to error

xtype/type.go

@@ -258,8 +258,14 @@ func (t *Type) UnescapedID() string {
 
 func (t *Type) asID(seeNamed, escapeReserved bool) string {
 	if seeNamed && t.Named {
-		pkgName := t.NamedType.Obj().Pkg().Name()
-		name := pkgName + t.NamedType.Obj().Name()
+		pkg := t.NamedType.Obj().Pkg()
+		name := t.NamedType.Obj().Name()
+		switch {
+		case pkg != nil:
+			name = pkg.Name() + name
+		case escapeReserved:
+			name = "x" + name
+		}
 		return name
 	}
 	if t.List {