POC: components for ERT

Use this to not announce ecl_well as a dependency across all projects since symbols only end up in a unit test of opm-core.

@bska POC of what we discussed yesterday. this way the find rule can be synced across projects just fine.
>> What does this (`list(FIND)`) do if [`COMPONENTS` is undefined]
>
> works just fine [and already done through prerequisite files]

Sorry for being dense. Are you saying that, in general,
```cmake
List (FIND)
```
on an undefined input list is benign and does something reasonable (e.g., returns `-1` or something like that) or is that behaviour particular to `ERT_FIND_COMPONENTS` because that list is derived from the prerequisite file?