VFS-agnostic architecture

[mkaput]

trafficstars

Split FilesGroup into 4 traits/query groups:

1. InputGroup: FilesPrototype that takes all non-file-related inputs & interns (crate configs, cfg_sets, flags etc.) (crate cairo-lang-filesystem)
2. FilesPrototype which will be a trait for a query group that provides a file_content query (and probably nothing more) (crate cairo-lang-filesystem)
3. FsGroup: FilesPrototype which will just read file contents (crate cairo-lang-filesystem)
4. VfsGroup: FilesPrototype which will support file overrides and all the bells and whistles we need for LS (crate cairo-lang-language-server)

This task is spin off #5355