Packages with several `elm-version` fields in elm.json can be published

[rlefevre]

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The following published packages contain several different elm-version fields, one of them being invalid:

showell/elm-data-util/1.0.0: https://github.com/showell/elm-data-util/blob/1.0.0/elm.json

    "elm-version": "0.19.0 <= v < 0.20.0",
    "source-directories": [
        "src"
    ],
    "elm-version": "0.19.0",

alex-tan/loadable/1.0.0 https://github.com/alex-tan/loadable/blob/1.0.0/elm.json

    "elm-version": "0.19.0 <= v < 0.20.0",
    "source-directories": [
        "src"
    ],
    "elm-version": "0.19.0",