[EROFS]: cleanup and introduce prefetch feature

[salvete]

What this PR does / why we need it:

1. Remove the global variables _target and _source, and introduce struct liberofs_file to wrap the source and target files.
2. Introduce prefetch feature.

The corresponding accelerated-container-image PR: https://github.com/containerd/accelerated-container-image/pull/299

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?

[BigVan]

thanks