The build was complaining:
meson.build:660: WARNING: Your libc does not have the C.UTF-8 locale and no other
suitable UTF-8 fallback locale could be found. You can still build GJS, but
some tests will fail.
The tests do not actually seem to be affected but still.
* Do not propagate spidermonkey, it is only needed gjs to link against.
* Split out dev output, any propagated dependencies should go there.
* Drop unnecessary sed, the readline thing is no longer there.
* Add passthru.tests to allow running installed tests easily.
* Drop pango from buildInputs, we only need it as a transitive dependency of gtk3 for installed tests; gjs actually depends on cairo.
* Move libxml2 to nativeBuildInputs, it is called by glib-compile-resources when xml-stripblanks preprocessing is used.
* Propagate glib, as it is in Requires in the pc file.
It is just 2.5 MB closure reduction but every megabyte counts.
