The `spack.architecture` module contains an `Arch` class that is very similar to `spack.spec.ArchSpec` but points to platform, operating system and target objects rather than "names". There's a TODO in the class since 2016:
abb0f6e27c/lib/spack/spack/architecture.py (L70-L75)
and this PR basically addresses that. Since there are just a few places where the `Arch` class was used, here we query the relevant platform objects where they are needed directly from `spack.platforms`. This permits to clean the code from vestigial logic.
Modifications:
- [x] Remove the `spack.architecture` module and replace its use by `spack.platforms`
- [x] Remove unneeded tests
319ae9254e