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/interpreters/python/hooks/unittest-check-hook.sh

@@ -13,5 +13,5 @@ unittestCheckPhase() {
 
 if [ -z "${dontUseUnittestCheck-}" ] && [ -z "${installCheckPhase-}" ]; then
     echo "Using unittestCheckPhase"
-    preDistPhases+=" unittestCheckPhase"
+    appendToVar preDistPhases unittestCheckPhase
 fi