WIP: move reader to its own package

Simple way of resolving the filename requested. Remove redirection logic from resolver itself.

For reference:

PoC branch https://gitlab.com/gitlab-org/gitlab-pages/-/blob/421-extract-to-use-in-zip/internal/serving/fileresolver/fileresolver_zip_test.go with tests for resolving files inside a zip archive.

Closes #421 (closed)