Problems with project.extension

This rule currently has many issues:

- The extension project inherits some things from Jamroot. I don't understand why.
- Extension projects are not usable from config files (even though the documentation says that extension projects can be initialized with using). This is a result of inheriting things from Jamroot, which doesn't exist yet when the config files are loaded.
- project.extension searches for the project root incorrectly (it skips project-config.jam). This breaks down if there is a project-config.jam above Jamroot, instead of alongside Jamroot.
- Is it even needed in the first place? standalone projects (with no location) work just fine.
- The "location" that it uses is very strange and often doesn't actually exist. If it does exist, then the behavior is likely to be surprising.
- It does most of its work in the caller's module. Only the `project` rule actually needs to be run in the caller's module.