Bug found by unused-but-set-variable, entry should be updated

Verfied by DeadYak

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: More memory management fixes.

DwarfImageDebugInfo:
- Type name/info entries weren't being freed properly.

BaseUnit:
- The array of entries itself was being destroyed, but not the actual
objects. Consequently none of the DIE objects were ever being freed.

CfaRuleSet:
- The register rules array wasn't being freed.

FileManager:
- References to entries weren't being released. Some more work remains to be
done here though, as a subset of these objects still have a non-zero ref
count in the end.

Debugger: Further work on memory leak hunting.

TeamWindow:
- Properly delete stack frame selection entries in destructor.

TeamDebugger:
- The reference to TeamDebugInfo wasn't correctly initialized to own,
leading to TeamDebugInfo never being destroyed.

FunctionInstance:
- Also clear source code reference in destructor.

ImageDebugInfo:
- Release references to specific infos in destructor. These are acquired
on our behalf when loading the image info, but were never released,
leading to the latter never being freed.

DwarfManager:
- Release references to files in destructor.

FileManager:
- Release entries in various destructors.

Somewhat improves the situation in #13800, but is still far from complete,
as a thorough review of this nature hasn't ever really been done.

libdebugger: Fix #12944.

LocatableFile:
- If there is no parent path, don't insert a path separator between parent
and filename. This may be the case depending on how the source file was
specified during compilation.

FileManager:
- When constructing an EntryPath from a LocatableEntry, ensure that the
parent folder actually has a path string that isn't simply empty to ensure
consistency with the raw dir/file case. Otherwise, hash lookups that are
dependent on the parent dir being NULL if not specified will fail, causing
us to not locate the file successfully. This was preventing us from updating
source location information for make 4.2's main.c, as the latter was
specified in such a way that the above combination of conditions would
occur, and consequently when asking the FileManager to update the source
location with the actual file, the entry couldn't be found in the table,
and no information would be updated.

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.