Fix module paths (#1502)

See https://go.dev/doc/modules/version-numbers#major-version.

## Buildtools PR checklist

- [x] The code in this PR is covered by unit/integration tests.
- [x] I have tested these changes and provide testing instructions
below.
- [x] I have either responded to, or resolved all Gemini comments on the
PR.
- [x] I have read Google Eng Practices on [Small
Changes](https://google.github.io/eng-practices/review/developer/small-cls.html),
this PR either follows these guidelines or the description provides
reasoning for why they can not be followed.

## Description

Work towards https://github.com/bazel-contrib/buildtools/issues/1237

According to https://go.dev/doc/modules/version-numbers#major-version,
the module path must include the major version for versions above v1.