Refactor "dump" sub-command (#30240)

Major changes:

* Move some functions like "addReader" / "isSubDir" /
"addRecursiveExclude" to a separate package, and add tests
* Clarify the filename&dump type logic and add tests
* Clarify the logger behavior and remove FIXME comments

Co-authored-by: Giteabot <teabot@gitea.io>

Author: wxiaoguang