implement proposal for fetching paths and retaining structure of dirs and packages (#914)

* implement proposal

* update docs and replace py-env

* more docs

* suggested proposal

* update docs

* add to_file parameter

* remove comment from Makefile

* suggested improvements

* move tests from basic to py_config

* retain leading slash from the first path

examples/numpy_canvas_fractals.html

@@ -84,9 +84,13 @@
             "numpy",
             "sympy"
           ],
-          "paths": [
-            "./palettes.py",
-            "./fractals.py"
+          "fetch": [
+            {
+              "files": [
+                "./palettes.py",
+                "./fractals.py"
+              ]
+            }
           ]
         }
       </py-config>