Debugger: Fix #13939, more work on #13800.

- Fix various cases where OpenHashTables weren't being cleared properly.
- Fix various reference counting errors.
- Simplify FileManager reference handling.
- Fix bug in LocatableDirectory where the directory named '/' would have its
name returned as empty. This would lead to failed lookups for entries already
in the table, and ultimately corrupted the hash table when deleting unused
entries, leading to #13939. This was previously never noticed due to the
entries not being freed properly.
- AbbreviationTable wasn't clearing its entries.

Debugger: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.