fix: handle Bazel load statements in extension-check

[immanuwell]

What this PR does / why we need it:
tools/extension-check blows up on normal Bazel config files because the embedded Starlark runner does not handle load(...).

This strips top level load(...) statements before evaluation, returns regular errors instead of panics, and removes the README workaround. Pretty small fix, but it makes the tool usable out of the box.

Which issue this PR fixes:
N/A, this repo has issues disabled.

Special notes for your reviewer:
Repro before this patch:
go run ./tools/extension-check --envoy-extensions-build-config ./bazel/extension_config/extensions_build_config.bzl --proxy-extensions-build-config ./bazel/extension_config/extensions_build_config.bzl --ignore-extensions ./tools/extension-check/wellknown-extensions

Before:
panic: load not implemented by this application

After:
it loads the checked-in file and reaches the normal missing-extension output. so yeah, this is a real bug on a real file, not some weird edge case.

Checks:
go test ./tools/extension-check
go test ./... -run '^$'

[istio-policy-bot]

😊 Welcome @immanuwell! This is either your first contribution to the Istio proxy repo, or it's been
a while since you've been here.

You can learn more about the Istio working groups, Code of Conduct, and contribution guidelines
by referring to Contributing to Istio.

Thanks for contributing!

Courtesy of your friendly welcome wagon.

[istio-testing]

Hi @immanuwell. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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