[tools] Finish porting to bzlmod

[jwnimmer-tri]

Specifically, respell the "download buildifier release binaries from github" into something bzlmod is happy with.

While we're here, also remove custom upgrade scripting for bazel. Dependabot handles that upgrade automation for us now (see e.g. #100).

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This change is 

[jwnimmer-tri]

+@zachfang for review, please?

FYI @rpoyner-tri

[zachfang]

I will catch up on bzlmod a bit.

Reviewed 9 of 9 files at r1.
Reviewable status: 1 unresolved discussion (waiting on @jwnimmer-tri)

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tools/upgrade_helper.py line 48 at r1 (raw file):

    """Overwrites ``.bazelversion`` with the new version of bazel."""
    with open(".bazelversion", "w", encoding="utf-8") as f:
        f.write(f"{new}\n")

nit. I assume these two functions can be removed as well as they are only called by _upgrade_bazel().

Code quote:

def _get_current_bazel_version() -> str:
    """Parses ``.bazelversion`` for the old version of bazel."""
    with open(".bazelversion", encoding="utf-8") as f:
        lines = f.read().splitlines()
    (line,) = lines
    return line.strip()


def _write_bazel_version(new):
    """Overwrites ``.bazelversion`` with the new version of bazel."""
    with open(".bazelversion", "w", encoding="utf-8") as f:
        f.write(f"{new}\n")

[jwnimmer-tri]

Reviewable status: complete! all discussions resolved, LGTM from assignee zachfang (waiting on @zachfang)

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tools/upgrade_helper.py line 48 at r1 (raw file):

Previously, zachfang wrote…

nit. I assume these two functions can be removed as well as they are only called by _upgrade_bazel().

Right! Thanks.

[zachfang]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all discussions resolved, LGTM from assignee zachfang (waiting on @jwnimmer-tri)