Vanity imports are not checked on internal packages

[mx-psi]

The current CI checks vanity import comments on exported packages via porto. Internal packages and modules are not checked, meaning that one can accidentally introduce incorrect vanity import comments, which break some tools like gomobile.

To avoid breaking those builds there are two options:

1. remove vanity import comments on internal packages and add a check to avoid introducing more or
2. add vanity import comments on internal packages and add a check for these.

I opened jcchavezs/porto#11 to implement option (2) but I want to get confirmation from OpenTelemetry Go maintainers that they are okay with this solution.

[Aneurysm9]

I'm fine with option 2.