Handle existing file-based installs in dotnetup

We should define what the behavior of dotnetup should be when installing an SDK (or runtime) and there is already a dotnet root on the path that is not managed by dotnetup. This may be a common scenario in some Linux environments (such as codespaces) where a file-based version of dotnet is automatically installed locally.

We could install into the existing folder (which seems to be what currently happens), but we won't be able to handle uninstall or upgrade as well with that root since there will be items that we don't keep track of in the dotnetup manifest.

It may be best initially to not allow installing into existing dotnet root folders, though this might be a poor experience.

We might be able to scan the existing structure of the folder and record those assets in the manifest, and then we wouldn't automatically delete those assets later on.