BuildStream silently allows overriding list defined in include

When you define a list like

```
build-depends:
- foo.bst
- bar.bst
```

in include and then set same in element body, there will be silent possibly unexpected replace when user was supposed to instead append. I propose behaviour be slightly adjusted such that when you redefine list with above syntax in element, there will be a warning that can be made fatal. (implicit-list-replace or something else, I'm up to different names) To fix the warning, you would have to in element in this case state intent you want to replace existing list through

```
build-depends:
(=):
- foo.bst
- bar.bst
```

If the list is not defined before, then there will be no warning with the short-hand syntax.