[!823] Rewrite FindSuiteSparse to provide imported targets

Merge branch 'feature/find_suitesparse' into 'master'

ref:core/dune-common\>

### Summary

This MR adds imported-targets to the FindSuiteSparse.cmake module, following
modern cmake guidelines.

### Details

Instead of exporting include directories and libraries as variables, the find
modules exports an imported target that contains all the necessary information
to include and link SuiteSparse libraries. The module allows to search for
various SuiteSparse components, like UMFPACK or SPQR, and introduces an
imported target for each of them in the form: SuiteSparse::<component>.
Additionally the combined imported target SuiteSparse::SuiteSparse is defined
to link against all found components.

Difference to old FindSuiteSparse module: It searches for all possible
components and creates corresponding component targets. The collective target
SuiteSparse::SuiteSparse then only contains the requested components.

See merge request [!823]

  [!823]: gitlab.dune-project.org/core/dune-common/merge_requests/823