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>
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