fixes after scalapb upgrade

[darl]

Description

Build fixes for scala 2.12 and strict_deps enabled

Motivation

Build failures in my current project after last PR (#1026)

[darl]

I think it's a problem:

CI fails with dependency_tracking_method = 'default' and without explicit protobuf dependency;

my local build fails with dependency_tracking_method = 'high-level' and with explicit dependency from master.

[ittaiz]

1. Sorry for the delay. Corona times are wrecking a havoc in my availability.
2. I think that the best thing to do is to revert your change and then push a new commit that only adds the failure that you're seeing locally to CI. Then we can try and devise a solution.
   I'll try to do it now.

cc @Jamie5 I have a hunch that somewhere in the flag changes we lost some of the e2e tests we used to have

[ittaiz]

btw, worst case I think we'll be able to add the problematic dep to the ignore targets of scalapb_generator_lib.
Let's hope we wont' reach this.

[ittaiz]

Also right now dependency_tracking_method='default' means dependency_tracking_method='high-level'.
It would really help to get a repro or even just your full setup.
What toolchain are you using (one of the default ones or a custom ones), what scala_repositories and which target fails (I'm assuming it's the same one that's failing right now but would love to make sure).

[ittaiz]

btw, it would also help to see the exact failure

[ittaiz]

Closing in favor of #1030 which has a workaround for both issues.
Feel free to reopen if you have strong objections.

[ittaiz]

btw, this isn't time critical or anything, right?
Asking since I'd like to give Jamie time to review #1030 and we don't have anything newer than the scalapb upgrade so I'm guessing you can wait but would still like to verify

[darl]

I can wait for #1030 to be merged