directives: depends_on should not admit anonymous specs (#34368)

Writing a long dependency like:

```python
     depends_on(
         "llvm"
         "targets=amdgpu,bpf,nvptx,webassembly"
         "version_suffix=jl +link_llvm_dylib ~internal_unwind"
     )
```

when it should be formatted like this:

```python
     depends_on(
         "llvm"
         " targets=amdgpu,bpf,nvptx,webassembly"
         " version_suffix=jl +link_llvm_dylib ~internal_unwind"
     )
```

can cause really subtle errors. Specifically, you'll get something like this in
the package sanity tests:

```
    AttributeError: 'NoneType' object has no attribute 'rpartition'
```

because Spack happily constructs a class that has a dependency with name `None`.

We can catch this earlier by banning anonymous dependency specs directly in
`depends_on()`.  This causes the package itself to fail to parse, and emits
a much better error message:

```
==> Error: Invalid dependency specification in package 'julia':
    llvmtargets=amdgpu,bpf,nvptx,webassemblyversion_suffix=jl +link_llvm_dylib ~internal_unwind
```

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