gnu: Move search path JUPYTER_PATH.

python-jupyter-core provides the `jupyter` binary as well as generic
functions to discover the search path for all Jupyter subprojects. Thus
it should provide the search paths too.

* gnu/packages/python-xyz.scm (jupyter): From here…
(python-jupyter-core): …to here.
[native-search-paths]: Add JUPYTER_CONFIG_DIR, because some Jupyter
projects install config files to their etc/ directory and expect them to
be loaded automatically.

gnu/packages/python-xyz.scm

@@ -7388,6 +7388,15 @@ (define-public python-jupyter-core
     (arguments `(#:tests? #f))
     (propagated-inputs
      `(("python-traitlets" ,python-traitlets)))
+    ;; This package provides the `jupyter` binary and thus also exports the
+    ;; search paths.
+    (native-search-paths
+     (list (search-path-specification
+            (variable "JUPYTER_CONFIG_DIR")
+            (files '("etc/jupyter")))
+           (search-path-specification
+            (variable "JUPYTER_PATH")
+            (files '("share/jupyter")))))
     (home-page "https://jupyter.org/")
     (synopsis "Jupyter base package")
     (description
@@ -10879,10 +10888,6 @@ (define-public jupyter
        ("python-nbconvert" ,python-nbconvert)
        ("python-notebook" ,python-notebook)
        ("python-qtconsole" ,python-qtconsole)))
-    (native-search-paths
-     (list (search-path-specification
-            (variable "JUPYTER_PATH")
-            (files '("share/jupyter")))))
     (home-page "https://jupyter.org")
     (synopsis "Web application for interactive documents")
     (description