Drop xerrors since functionality is merged to errors in std lib

[arvenil]

Drop the only dependency since all functionality from xerrors used in go-cmp is now merged to errors in standarad library.

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[arvenil]

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[dsnet]

Thanks for the PR, but we can't accept this. The new errors API was added in go1.14, but our current minimally supported version is go1.8. The cmp package is very widely used, so we need to support a wide range of Go versions. It will be a long time before we get to go1.14 is the minimally supported version.

[arvenil]

Hi, makes sense, feel free to close it. However I'm suprised since people using older golang version can always download older go-cmp version 🤷.

[dsnet]

people using older golang version can always download older go-cmp version 🤷.

It's not quite that simple. Suppose there is module A at version v1.5 and that module depends on cmp at version v0.6 (with go1.14 as the minimum support).

If you're constrained to an old version of Go and want to use A@v1.5, you're stuck since it requires you to use cmp@v0.6, which requires that you use go1.14.

Closing this PR since we won't get to this point for at least a year or more.