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-remove-bin-bytecode-hook.sh

@@ -13,5 +13,5 @@ pythonRemoveBinBytecodePhase () {
 }
 
 if [ -z "${dontUsePythonRemoveBinBytecode-}" ]; then
-    preDistPhases+=" pythonRemoveBinBytecodePhase"
+    appendToVar preDistPhases pythonRemoveBinBytecodePhase
 fi