Like `spack change` for specs in environments, this can e.g. replace `examplespec+debug` with `examplespec~debug` in a `require:` section.
Example behavior for a config like:
```
packages:
foo:
require:
- spec: +debug
```
* `spack config change packages:foo:require:~debug` replaces `+debug` with `~debug`
* `spack config change packages:foo:require:@1.1` adds a requirement to the list
* `spack config change packages:bar:require:~debug` adds a requirement
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