Stop relying on `pkg/errors` to handle stack traces

Today the stack trace formatting code delegates to `pkg/errors/stack.go`.

However, that code uses `runtime.FuncForPC` and emits file paths in the stack trace derived from the raw file name of the source file.

This method is outdated, as the on-disk file name of a given package's source may be different from the package path (due to go mod versioning, reproducible build sandboxes, etc).

The "modern" equivalent is `runtime.CallersFrame`, which populates `runtime.Frame` structs with package path-qualified function names.

We want to use that instead.