Remove use of deprecated package pkg/errors

[lukemassa]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

Per, pkg/errors#245 github.com/pkg/errors is deprecated and read-only. Golang's standard library now has support for wrapping errors, so we can switch to that.

Another benefit if right now there's code that looks like:

		if !os.IsNotExist(err) {
			return valid.RepoCfg{}, errors.Wrapf(err, "unable to read %s file", repoConfigFile)
		}
		// Don't wrap os.IsNotExist errors because we want our callers to be
		// able to detect if it's a NotExist err.
		return valid.RepoCfg{}, err

Because later code calls os.IsNotExist on the error. The documentation for os.IsNotExist recommends using errors.Is(err, ErrNotExists, which can walk "up the chain" of a wrapped error, thus we can get rid of that entire block above.

Describe the solution you'd like
Eliminate pkg/errors as a dependency.

Describe the drawbacks of your solution
There are a lot of calls to errors.Wrapf() that will need to be changed, but I imagine it'll be pretty mechanical.
Another concern is a lot of merge conflicts because this code is all over the code base, hopefully we can find ways to chunk it up and/or do it quickly

Describe alternatives you've considered
There are other error wrapping packages we could use, but it seems like no reason not to just the one in the stdlib.