ConfigPath should be relative to something

Currently it's possible to get relative paths into the config schema.
Some things like sshfs do not like state_dir being relative, and there are all sorts of ways in which this could behave unexpectedly.
Other things like include and plugins are relative to the config file from which they are loaded.
It would be nice if ConfigPath was always relative to the config file it was loaded from,.
Dealing with that probably involves injecting something into base_injector or using a context variable to keep track of what to be relative to.
It also may be complicated depending on when things are resolved.