Debugger: don't auto-download packages non-interactively.

- When in non-interactive mode, i.e. saving a crash report, don't
allow the image debug info loader to automatically grab missing but
available info packages. Otherwise we potentially download very large
packages with the user being entirely unaware, i.e. the 200MB debug
information package now present by default for gcc7's libgcc. This
should eventually be made a configurable preference though.

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.