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/python-catch-conflicts-hook.sh

@@ -6,5 +6,5 @@ pythonCatchConflictsPhase() {
 }
 
 if [ -z "${dontUsePythonCatchConflicts-}" ]; then
-    preDistPhases+=" pythonCatchConflictsPhase"
+    appendToVar preDistPhases pythonCatchConflictsPhase
 fi