fix: `pip debug` cert configuration level detection (#14056)

ca_bundle_info reports which configuration level sets the cert value in
pip debug output. It derives the levels from config.items(), but since
#12201 the configuration dictionary is keyed by file name rather than by
section.option. The function still splits the outer key on ., so it
collects file-path fragments (e.g. /etc/pip from /etc/pip.conf) instead
of section names, never matches install/wheel/download, and always
reports global.