Dear Arlo,
   In his Sanskrit introduction to the first volume of the Nirnayasagara edition of Patañjali's VyÄkaraṇa-MahÄbhÄá¹£ya (1917, p. 16, fn 5), the editor Shivadatta Kuddala says that this verse is from ParÄÅ›ara-UpapurÄṇa. The same source is given for the traditional definitions of SÅ«tra and VÄrttika. (KuddÄla, Intro, p. 12 and 14). For the definition of SÅ«tra, KuddÄla also refers to Viṣṇūdharmottara (ibid, p. 12). On p. 11 of his Introduction, KuddÄla quotes the following verse from ParÄÅ›ara-UpapurÄṇa:
   pÄṇinÄ«yaṃ mahÄÅ›Ästraṃ padasÄdhutvalaká¹£aṇam /
   sarvopakÄrakaṃ grÄhyaṃ ká¹›tsnaṃ tyÄjyaṃ na kiṃcana //
From this it appears that there is a section in the ParÄÅ›ara-UpapurÄṇa dealing with PÄṇini, and the definitions of SÅ«tra, VÄrttika and BhÄá¹£ya likely appear in that section.
  I don't have a copy of ParÄÅ›ara-UpapurÄṇa with me at hand, but that is one possible source to check. Hope this helps. Best,
Madhav Deshpande