Fix mock generation of function pointer signature parameters (#227)

Before this fix an interface method with a parameter such as
`func(goa.Endpoint)` would be generated as
`func(goa.design/goa/v3/pkg.Endpoint)` instead of something that is
actually valid Go.

mock/cmd/cmg/pkg/generate/_tests/extensive/extensive.go

@@ -36,6 +36,7 @@ type (
 			C    float64
 		}, func(int) (bool, error))
 		NamedTypes(Struct, Array, io.Reader, imported.Type, goa.Endpoint, Generic[uint, string, Struct, Array]) (Struct, Array, io.Reader, imported.Type, goa.Endpoint, Generic[uint, string, Struct, Array])
+		FuncNamedTypes(func(Struct, Array, io.Reader, imported.Type, goa.Endpoint, Generic[uint, string, Struct, Array])) func(Struct, Array, io.Reader, imported.Type, goa.Endpoint, Generic[uint, string, Struct, Array])
 		VariableConflicts(f, m uint)
 	}
 

mock/cmd/cmg/pkg/generate/type.go

@@ -82,7 +82,10 @@ func (ta *typeAdder) name(tt types.Type) (name string) {
 			name = "*" + ta.name(t.Elem())
 		case *types.Signature:
 			b := &bytes.Buffer{}
-			types.WriteSignature(b, t, nil)
+			types.WriteSignature(b, t, func(p *types.Package) string {
+				i := addImport(newImport(p.Path(), p.Name()), ta.stdImports, ta.extImports, ta.intImports, ta.modPath)
+				return i.AliasOrPkgName()
+			})
 			name = "func" + b.String()
 		case *types.Slice:
 			name = "[]" + ta.name(t.Elem())

mock/cmd/cmg/pkg/version.go

@@ -10,7 +10,7 @@ const (
 	// Minor version number
 	Minor = 17
 	// Build number
-	Build = 0
+	Build = 1
 	// Suffix - set to empty string in release tag commits.
 	Suffix = ""
 )