gh-136073: Fix expandvars() in Tools/freeze/checkextensions.py

[serhiy-storchaka]

• Fix potential infinite recursion and/or memory consumption.
• Fix a bug when reference can cross boundaries of substitutions, e.g. expandvars('$a)', {'a': '$(b', 'b': 'c'}).
• Fix potential quadratic complexity.

• Issue: gh-136073