Specifying port for feature registry is broken

[MihaiBSony]

Features that come from registries with explicit port numbers are sometimes ignored, with only the last one remaining. For example, if devcontainer.json contains

"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)

[joshspicer]

Thanks for this report. I'll comment on your PR.