using remote vs local opam-repository during build

[amirmc]

In #462 (which is relevant to #463)

@samoht asked: "Why not using https://github.com/ocaml/opam-repository.git as a repository instead of a local one ?"

@pw374 suggested: "I guess it’s because you don’t want to clone the whole repository each time the list is generated, and instead just get the diff, also if you use a local repo it's easier to decide whether you need to update your list. However I don't know that well the opam2web part. @dsheets any thoughts?"