Reflect in Current Directory First (#390)

Some users have intention to run mockgen in a module aware directory with
version pinned to those in `go.mod`.

Always try to run reflect program in current working directory first.

mockgen/reflect.go

@@ -129,6 +129,7 @@ func runInDir(program []byte, dir string) (*model.Package, error) {
  if err := cmd.Run(); err != nil {
  return nil, err
  }
+
  return run(filepath.Join(tmpDir, progBinary))
 }
 
@@ -152,6 +153,11 @@ func reflectMode(importPath string, symbols []string) (*model.Package, error) {
 
  wd, _ := os.Getwd()
 
+ // Try to run the reflection program in the current working directory.
+ if p, err := runInDir(program, wd); err == nil {
+ return p, nil
+ }
+
  // Try to run the program in the same directory as the input package.
  if p, err := build.Import(importPath, wd, build.FindOnly); err == nil {
  dir := p.Dir
@@ -160,11 +166,7 @@ func reflectMode(importPath string, symbols []string) (*model.Package, error) {
  }
  }
 
- // Since that didn't work, try to run it in the current working directory.
- if p, err := runInDir(program, wd); err == nil {
- return p, nil
- }
- // Since that didn't work, try to run it in a standard temp directory.
+ // Try to run it in a standard temp directory.
  return runInDir(program, "")
 }