Remove deprecated module systems es6 and es6-global

[cknitt]

• Removes the deprecated module systems es6 and es6-global (superseded by esmodule)
• Adds proper validation of module systems to rewatch
• Removes unused stuff from Ext_path

Supersedes #8130

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commit: a1924ee

[cristianoc]

Great.
In the error message when a legacy mode is used, perhaps suggest what to use instead.

[cknitt]

Great. In the error message when a legacy mode is used, perhaps suggest what to use instead.

I don't know, the corresponding deprecation warning with migration instructions was already in v12.

Now there is no special handling anymore, and the error says

"Module system \"{other}\" is unsupported. Expected \"commonjs\" or \"esmodule\"."

It should still be clear enough what to migrate to from "es6" even if one skipped v12.