python3: hide -src package until main pkg selected

This adds a 'Package/<pkg>-src/config' definition with a 'depends on
<pkg>' line, which will hide <pkg>-src unless <pkg> is selected.  This
makes the long list of python packages a bit shorter, and also indents
the src package:

<M> python3-base................................ Python 3.8 interpreter
< >   python3-base-src................. Python 3.8 interpreter (sources)

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

lang/python/python3-package.mk

@@ -56,6 +56,10 @@ define Py3Package
     (Contains the Python3 sources for this package).
   endef
 
+  define Package/$(1)-src/config
+    depends on PACKAGE_$(1)
+  endef
+
   # Add default PyPackage filespec none defined
   ifndef Py3Package/$(1)/filespec
     define Py3Package/$(1)/filespec