Start migrating to Intel oneMKL

[albestro]

Fixes #878 Close #626

This PR starts with introducing logic for using Intel OneAPI MKL (or oneMKL), which now provides an official CMake Config file. From discussions with @rasolca, for the moment, we opted for keeping also the "legacy" custom module.

• DLAF_WITH_MKL triggers the new logic using official oneMKL CMake config file;
• DLAF_WITH_MKL_LEGACY triggers the "legacy" logic which uses our custom CMake module.

_note: currently, DLAF_WITH_MKL_LEGACY seems to work fine also for oneAPI MKL installation._

Spack

The spack package has been updated the same way. The logic could be simplified by fully relying on spack (have a look here) and just use the "link-line" logic as any other BLAS/LAPACK library would do. This would require a change, since DLAF_WITH_MKL is used not just for linking purpose but also for implementation details like mkl_set_num_threads. Anyway, this is not needed now and we might evaluate in the future (e.g. when MKL legacy branch will be dropped).

note: intel-parallel-studio is not supported (to my knowledge it has been deprecated too and merged into intel-oneapi)

CI

In the CI nothing has changed yet. It still installs intel-mkl as external package, and then spack uses it. The package should™️ deal correctly with it by using DLAF_WITH_MKL_LEGACY. Switching to intel-oneapi-mkl is outside the objectives of this PR (~I will open an issue to track this and decide what to do~ see #1074).

TODO

• [x] check if #626 can be closed by this PR

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6ef1381) 94.06% compared to head (bf3a17f) 94.06%. Report is 5 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1073 +/- ## ======================================= Coverage 94.06% 94.06% ======================================= Files 148 148 Lines 9199 9199 Branches 1164 1164 ======================================= Hits 8653 8653 Misses 323 323 Partials 223 223 ```

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