Migrate XLS OSS build to full Bzlmod and Bazel 8.7.0.

[copybara-service]

Migrate XLS OSS build to full Bzlmod and Bazel 8.7.0.

This CL completes the migration of the open-source XLS build system to Bzlmod and Bazel 8.7.0, moving the final rules_hdl dependency to MODULE.bazel.

Fixes #931.

Key changes:

• Migrated all open-source dependencies (including rules_hdl and OpenROAD) to Bzlmod in MODULE.bazel, completely removing legacy WORKSPACE linkages.
• Upgraded the required Bazel version to 8.7.0.
• Disabled legacy WORKSPACE evaluation in xls.bazelrc by removing the --enable_workspace flag, while preserving a minimal WORKSPACE file to maintain compatibility with older Bazelisk and IDE root-detection tools.
• Upgraded OpenROAD to a recent main commit (1ac310f9) to pull in upstream Bzlmod compatibility fixes.
• Configured rules_pycross in MODULE.bazel to only target the active Python version, preventing build failures on host systems without Python 3.8.
• Upgraded or-tools to 9.15 to align with OpenROAD's dependency requirements, and patched it to restore public visibility to pdlp_solver to resolve a target analysis block.
• Updated Copybara translation rules in copy.bara.sky to correctly map the new open-source OpenSTA targets and translate runfile paths for FDO tools (Yosys, OpenSTA, ABC) to their Bzlmod-compliant canonical names.
• Cleaned up obsolete patches and unnecessary Bzlmod single-version overrides.