Break module extension groups after a `use_repo` (#1339)

This allows users to separate tags and associated `use_repo` statements into separate groups.

Also use unified diffs in the integration tests since ordinary diffs make it very hard to reason about whitespace.

Before:
```
go_deps = use_extension("//:extensions.bzl", "go_deps")
go_deps.from_file(go_mod = "//:go.mod")
use_repo(
    go_deps,
    "com_github_bazelbuild_buildtools",
)
go_deps.module(name = "foo")
use_repo(go_deps, "foo")
```

After:
```
go_deps = use_extension("//:extensions.bzl", "go_deps")
go_deps.from_file(go_mod = "//:go.mod")
use_repo(
    go_deps,
    "com_github_bazelbuild_buildtools",
)

go_deps.module(name = "foo")
use_repo(go_deps, "foo")
```