Refactored packm variants.

Details:
- Revised packm_blk_var2() and _var3() by encapsulating the general,
  hermitian/symmetric, and triangular panel-packing subproblems into
  separate functions: packm_gen_cxk(), packm_herm_cxk(), and
  packm_tri_cxk(), respectively. Also, homogenized the packm code as
  well as the new specialized packm_*_cxk() code to further improve
  readability.