Error when an anonymous spec is required for a virtual package (#49385)

When requiring a constraint on a virtual package, it makes little
sense to use anonymous specs, and our documentation shows no example
of requirements on virtual packages starting with `^`.

Right now, due to how `^` is implemented in the solver, writing:
```yaml
mpi:
  require: "^openmpi"
```
is equivalent to the more correct form:
```yaml
mpi:
  require: "openmpi"
```
but the situation will change when `%` will shift its meaning to be a
direct dependency.

To avoid later errors that are both unclear, and quite slow to get to the user,
this commit makes anonymous specs under virtual requirements an error,
and shows a clear error message pointing to the file and line where the
spec needs to be changed.

Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

share/spack/gitlab/cloud_pipelines/stacks/data-vis-sdk/spack.yaml

@@ -25,7 +25,7 @@ spack:
       # Minimize LLVM
       variants: ~lldb~lld~libomptarget~polly~gold libunwind=none compiler-rt=none
     libllvm:
-      require: ["^llvm"]
+      require: ["llvm"]
     visit:
       require: ["@3.4.1"]