Switch from `git_fetch@{remote,commit,reference}` to `git_fetch@urls`

Allow supporting multiple URLs endpoints.

We will eventually want to support bundle endpoints via https://

We can use the URL idea of anchors to specify the committish:

git_fetch(
    urls = [
        # TODO: implement: "https://gitlab.arm.com/.../git-2.50.0.bundle",
        "https://github.com/git/git.git#refs/tags/v2.50.0",
        "git@github.com:git/git.git#16bd9f20a403117f2e0d9bcda6c6e621d3763e77",
    ],
)

This will remove remote, commit and reference attributes.

We will need a simple parser that converts the URL into remote, commit, reference underneath.