field 'requires', expression did not reference any resources too strict

### Enhancement Proposal

Say you have a resource `foo`:

```yaml
var: foo

var: bar
```

and a templated unit:

```yaml
unit: template
template-resource: foo
id: foo/{var}
requires:
"{var}" != "bar"
```

This will result in a validation error:
```
error: units/foo.pxu:5: job 'foo/FUNCTION', field 'requires', expression did not reference any resources
```

So you need to invent an expression to `and` with it. But it's a perfectly valid expression, IMO.

Start by locating the validation logic for the `requires` field and reproduce the YAML example from the issue. Determine why an expression without a resource reference is rejected; done means the shown expression is accepted without an invented condition and a regression test covers the case.

Written by the indexing model from the issue text.