[plugins] Always create plugin dir (#858)

## Summary

We were not creating the plugin dir if there were no files in it but the
redis plugin depends on the directory existing. Solution is to always
create directory.

Fixes #856

## How was it tested?

```
devbox add redis
devbox services start redis
```

Author: mikeland73

internal/plugin/plugin.go

@@ -49,6 +49,11 @@ func (m *Manager) CreateFilesAndShowReadme(pkg, projectDir string) error {
 		return nil
 	}
 
+	// Always create this dir because some plugins depend on it.
+	if err = createDir(filepath.Join(projectDir, VirtenvPath, pkg)); err != nil {
+		return err
+	}
+
 	debug.Log("Creating files for package %q create files", pkg)
 	for filePath, contentPath := range cfg.CreateFiles {
 		if !m.shouldCreateFile(filePath) {