fix and enhance dynamic prefetcher

[WaberZhuang]

What this PR does / why we need it:

• Prefetcher should use the readonly lower_file but not the m_file, or some error will occur when using upper layer and dynamic prefetcher together.
• Fix the way to resolve filepath, now we could use '/' as a prefetch item.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?