Make MLIR Python sources idempotent by moving the custom commands off
the custom target for each source.
Previously, a custom target was created for each MLIR Python source file
and a custom command added to symlink it to a destination path. However
this causes the build to not be idempotent because custom targets always
run.
Instead, this PR separates the symlink/copy to destination paths into
custom commands and makes the custom target depend on the output. That
prevents re-running them and restores idempotency.
Testing:
- Build with `-D MLIR_ENABLE_BINDINGS_PYTHON=ON`. Prior to this change,
the build is not idempotent (building twice re-runs the symlink/copy
command for Python source targets). After this change it is idempotent.
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