Performance boost: reduce instantiations of re.Scanner

- Lexer is the same for every Spec parser in spack, so don't build it
  every time.

- This improves time to import package.py files a lot, as a Lexer
  doesn't have to be constructed for every spc in the packages.

- To concretize dealii:
  - Before: ~20 sec
  - After:  ~6 sec