Mark `toolchains` extension as reproducible

Copybara Import from https://github.com/bazelbuild/rules_java/pull/187

Needed a manual import with a copybara fix (top-level targets for transformed repositories need special casing)

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Mark `toolchains` extension as reproducible (#187)

This avoids an unnecessary and verbose lockfile entry for rules_java users. Uses `bazel_skylib`'s `modules.as_extension` to automatically detect the availability of the `reproducible` feature.

Closes #187
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PiperOrigin-RevId: 633475117
Change-Id: I05f1578e767f2148ef59a041c1b505c30976c779