Clean up sparse handling

[ChrisRackauckas]

• Fix case with sparse b, fixes https://github.com/SciML/LinearSolve.jl/issues/248
• Removes Sparspak from the section off code that's only for non-GPL cases. Sparspak.jl isn't GPL-licensed so we can always use it.
• Restructured the default algorithm so that Sparspak is used when GPL is disabled, making it so it can always factorize properly.

[codecov[bot]]

Codecov Report

Merging #249 (4a6c243) into main (64200a1) will decrease coverage by 0.48%. The diff coverage is 96.29%.

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
- Coverage   65.81%   65.32%   -0.49%     
==========================================
  Files          12       12              
  Lines         740      744       +4     
==========================================
- Hits          487      486       -1     
- Misses        253      258       +5     

Impacted Files	Coverage Δ
src/default.jl	46.75% <0.00%> (-1.30%)	:arrow_down:
src/factorization_sparse.jl	100.00% <ø> (ø)
src/common.jl	89.83% <100.00%> (+0.73%)	:arrow_up:
src/factorization.jl	77.87% <100.00%> (+0.19%)	:arrow_up:

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more