Cherry-picking virtual dependencies (#35322)

This PR makes it possible to select only a subset of virtual dependencies from a spec that _may_ provide more. To select providers, a syntax to specify edge attributes is introduced:
```
hdf5 ^[virtuals=mpi] mpich
```
With that syntax we can concretize specs like:
```console
$ spack spec strumpack ^[virtuals=mpi] intel-parallel-studio+mkl ^[virtuals=lapack] openblas
```

On `develop` this would currently fail with:
```console
$ spack spec strumpack ^intel-parallel-studio+mkl ^openblas
==> Error: Spec cannot include multiple providers for virtual 'blas'
    Requested 'intel-parallel-studio' and 'openblas'
```

In package recipes, virtual specs that are declared in the same `provides` directive need to be provided _together_. This means that e.g. `openblas`, which has:
```python
provides("blas", "lapack")
```
needs to provide both `lapack` and `blas` when requested to provide at least one of them.

## Additional notes

This capability is needed to model compilers. Assuming that languages are treated like virtual dependencies, we might want e.g. to use LLVM to compile C/C++ and Gnu GCC to compile Fortran. This can be accomplished by the following[^1]:
```
hdf5 ^[virtuals=c,cxx] llvm ^[virtuals=fortran] gcc
```

[^1]: We plan to add some syntactic sugar around this syntax, and reuse the `%` sigil to avoid having a lot of boilerplate around compilers.

Modifications:
- [x] Add syntax to interact with edge attributes from spec literals
- [x] Add concretization logic to be able to cherry-pick virtual dependencies
- [x] Extend semantic of the `provides` directive to express when virtuals need to be provided together
- [x] Add unit-tests and documentation

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