Julia: Rename/restructure SuiteSparse

[andreasnoack]

It's not great to ask users so run using SuiteSparse before they can solve a sparse linear system. Ideally, most users shouldn't need to know what SuiteSparse is in the same way as we don't require them to know about LAPACK to do dense linear algebra. Either, we could make the sparse solvers part of SparseLinearAlgebra module or load them as part of the SparseArrays module.

[KristofferC]

Loading SuiteSparse automatically if it exists when loading SparseArrays makes sense. We could say that SuiteSparse is a conditional dependency to SparseArrays ;)

[ViralBShah]

We should load all the sparse matrix stuff, sparse solvers, eigensolvers with using LinearAlgebra.

[ViralBShah]

The plan is to move it out of stdlib, so not a matter to be discussed further here.