treewide: handle preDistPhases __structuredAttrs-agnostically

Always specify the preDistPhases attribute as a list instead of a string.

Append elements to the preDistPhases Bash variable using appendToVar
instead of string or Bash array concatenation.

Handle element insertion before a specific element using string
substitution as before, but handle both structured and unstructured
attributes.

pkgs/development/python2-modules/pytest/default.nix

@@ -42,7 +42,7 @@ buildPythonPackage rec {
         find $out -name .pytest_cache -type d -exec rm -rf {} +
     }
 
-    preDistPhases+=" pytestcachePhase"
+    appendToVar preDistPhases pytestcachePhase
 
     # pytest generates it's own bytecode files to improve assertion messages.
     # These files similar to cpython's bytecode files but are never laoded
@@ -55,7 +55,7 @@ buildPythonPackage rec {
         #    https://github.com/pytest-dev/pytest/blob/4.6.11/src/_pytest/assertion/rewrite.py#L32-L47
         find $out -name "*-PYTEST.py[co]" -delete
     }
-    preDistPhases+=" pytestRemoveBytecodePhase"
+    appendToVar preDistPhases pytestRemoveBytecodePhase
   '';
 
   meta = with lib; {