You can define a new importer object by importing a subdirectory in GJS.
This is undocumented, but it is likely to at least hold until the whole
thing moves to ES6 modules, after which we'll be able to do this purely
in JS with Reflect.Loader.
Since this was the only thing the ShellJS library did, we can remove it
altogether.
This allows us to discontinue use of the gjs-internals-1.0 embedder API.
https://bugzilla.gnome.org/show_bug.cgi?id=772386
Since commit 1ebb162a00 moved JS sources into resources,
the extension-prefs tool was broken. To fix it, we would either
need to generate an external GResource in addition to the generated
C code and teach gjs-console about loading it before evaluating
the script, or turn gnome-shell-extension-prefs into a binary with
the JS resources compiled in.
https://bugzilla.gnome.org/show_bug.cgi?id=722334
