Amend SE-0364 to make the wording stronger

[harlanhaskins]

This removes "considered" and expands a bit on why same-package conformances will be non-retroactive, once duplicate conformances are able to become link errors.

[harlanhaskins]

I can amend further if we don't think the duplicate symbol will be detectable at link time, and will instead be detected at load-time.

[tshortli]

In the following package structure, where B and C are dylibs and the dependency from B to C is internal/@_implementationOnly, overlapping conformances in A and C can only be detected at load time I think:

graph LR;
    A-->B;
    B-->C;

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