swag init generates documentation too slowly

[luoxianjie]

Is your feature request related to a problem? Please describe.
As our projects become larger and larger, swag init will be very slow to generate documents, usually taking more than half an hour, which is very painful.

Describe the solution you'd like
Looking at the source code, we found that it takes a lot of time to search for dependency packages, because all dependencies will be searched repeatedly. It is not as good as both B and C depending on A. When building B and C, A will be searched repeatedly. We add the FindPackage method. After add the cache, I found that the speed increased by nearly 10 times. The code is as follows

conf := loader.Config{
  ParserMode:  goparser.ParseComments,
  Cwd:         cwd,
  FindPackage: FindPackage,
}

var packageCache = make(map[string]*build.Package)
var cacheMutex sync.RWMutex
var packageLocks sync.Map

func FindPackage(ctxt *build.Context, importPath, fromDir string, mode build.ImportMode) (*build.Package, error) {
	cacheMutex.RLock()
	pkg, found := packageCache[importPath]
	cacheMutex.RUnlock()

	if found {
		return pkg, nil
	}

	lockInterface, _ := packageLocks.LoadOrStore(importPath, &sync.Mutex{})
	lock := lockInterface.(*sync.Mutex)

	lock.Lock()
	defer lock.Unlock() 

	cacheMutex.RLock()
	pkg, found = packageCache[importPath]
	cacheMutex.RUnlock()

	if found {
		return pkg, nil
	}

	pkg, err := ctxt.Import(importPath, fromDir, mode)
	if err != nil {
		return nil, err
	}

	cacheMutex.Lock()
	packageCache[importPath] = pkg
	cacheMutex.Unlock()

	return pkg, nil
}