In preparation for publishing a separate module of the Go SDK for Pulumi
on which providers can depend, we should reduce the dependency footprint
so as to cause end users as few issues as possible with transitive
dependency versioning.
This commit removes all use of `github.com/pkg/errors` from the Go SDK
to that end, replacing it with the standard `errors` package and `fmt`
for error formatting where appropriate. We use the new (as of Go 1.13)
"%w" syntax for wrapping errors, so this code is no longer compatible
with Go 1.12.
666c5d5cdc