libdebugger: Reorganize debugger_interface subdir.

- Split into separate subdirectories for each interface type, as is done in
the target_host_interface subdir. Preparation for the remote interfac

libdebugger: Reorganize debugger_interface subdir.

- Split into separate subdirectories for each interface type, as is done in
the target_host_interface subdir. Preparation for the remote interfaces,
which will have quite a few more components than the existing local ones.

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