Specifying port for feature registry is broken

Features that come from registries with explicit port numbers are sometimes ignored, with only the last one remaining. For example, if devcontainer.json contains
```json
"features": {
"localhost:5000/name/repo/feature1:1": {},
"localhost:5000/name/repo/feature2:1": {}
}
```
only `feature2` will be installed. This happens because at some places in the CLI code the features are indexed by "whatever is before the first `:` character" (running `grep -rn "split(':')\[0\]"` on the repo shows multiple locations where this happens)