gnu: Reference the inherited inputs values.

This commit was made by running this command:

  sed -e's/modify-inputs (package-\([a-z-]*\)inputs [a-zA-Z0-9-]\+)/modify-inputs \1inputs/g' -i gnu/packages/*.scm

… and then reverting individual hunks where the change would trigger unbound
variable warnings or other issues (such as ‘native-inputs’ is bound in the
body of the ‘inputs’ field, but it refers to the ‘native-inputs’ thunk defined
just above).

Change-Id: I6d94819f2809313fa1fbefc61897502ee7d66fab

gnu/packages/python.scm

@@ -661,7 +661,7 @@ def contents() -> str:
      (modify-inputs (package-inputs python-2.7)
        (replace "openssl" openssl)))
     (native-inputs
-     (let ((inputs (modify-inputs (package-native-inputs python-2)
+     (let ((inputs (modify-inputs native-inputs
                      (prepend tzdata-for-tests
                               unzip
                               (@ (gnu packages compression) zip)))))