phase out "magento-module" by introducing "openmage-module" as new default

[Flyingmana]

with being primarily used by OpenMage, the community fork of Magento1, it makes sense to align the branding here.

The old name will still be supported for ever (because it cant hurt)

Refers mostly to the composer package type as in

{
    "type": "magento-module",
    ...
    "extra": {
        "magento-root-dir": "htdocs/",
        "include-root-package": true
    }
}