Switch to Go native error wrapping

[kolyshkin]

• switch to native error wrapping
• add go.mod
• update go versions in travis.yml

Compatibility notes:

1. ErrSymlinkLoop is removed in favor of wrapping syscall.ELOOP into
   &os.PathError{}; users should use e.g. errors.Is(err, syscall.ELOOP)
   to check for this particular error.

2. IsNotExist no longer use pkg/errors.Cause but native go error
   unwrapping; it could breaks a use case when it is used for some
   third-party errors wrapped using pkg/errors.Wrap[f].

I was not able to find any users of either feature.

This is a shorter/simpler alternative to #4. The differences are:

• no intermediate vendoring of pkg/errors;
• slightly simpler IsNotExist implementation;
• removing ErrSymlinkLoop, using os.PathError wrapping to return ELOOP.

@thaJeztah @cyphar PTAL

[cyphar]

LGTM. 👍 to dropping everything in one go -- thanks for pushing this originally @thaJeztah!

[thaJeztah]

Thanks!