[VFS] Add a vfs package

[jinroh]

This PR is only refactorization. The main idea is to export the vfs parts in its own package to give us some space for what is coming next.

The diff may be hard to read so here exact list of what it does:

• create a package vfs with all the code specific to vfs, file and directory handling (no gin)
• add error handling in vfs/errors
• renamed vfs.DocMetadata struct into vfs.DocAttributes and make its fields private
• add mime, class and size fields to vfs.DocAttributes to be more consistent
• use web/errors in web/files to be consistent with web/data

[aenario]

👍 LGTM, in the future we can imagine having some of the parsing (MD5 & content-length) moved to middlewares.