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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