nogo: ignore generated source files

[sluongng]

Under GoCompilePkg actions, there are several ways that source files
could have been modified / generated that is out of end users
control:

• go_test would generate an '_empty.go' file for the missing
  internal/external package to compile it.

• coverage instrumentation would rewrite test source files with a
  wrapper source file that collects coverage data into a determined
  variable.

• CGO usage would transform the original source files into several
  output Go and C source files with go tool cgo ...

Implement a mechanism to ignore the generated source files for the first
2 cases and instruct static analyzers to run over the original source
files wherever applicable. This provides a better default for rules_go
users as static analysis errors over these generated files are outside
of their control and thus, could not be fixed.

Add a note regarding the CGO case for future investigations.

Replacement of #2996
Fixes #2984

[google-cla]

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[sluongng]

See #2518 for details

[sluongng]

The reason why I skip on doing CGO is:

• It's a bit more complex to dive into how cgo2() work
• I have yet to encounter a CGO generated code conflicting with static analyzers myself.

Would love if somebody could address that in the future as I believe it would make the code a lot 'cleaner'.

[fmeum]

Not dealing with CGo code until there is a clear need to do so seems fine to me.

[sluongng]

@fmeum I applied your comment suggestion but Google's CLA does not seems to have the email that is used in the commit trailer:

The following contributors were found for this pull request:

✅ https://github.com/bazelbuild/rules_go/commit/de2e09007e5d3e354ae12d8b042ca273edd790db Author: @sluongng <sl****ng​@gmail.com>
❌ https://github.com/bazelbuild/rules_go/commit/de2e09007e5d3e354ae12d8b042ca273edd790db Co-Author: <fa***n​@meumertzhe.im>

If you don't want to sign the CLA w. that email, I can either do an amend to change to an email of your choice or to remove the Co-Author trailer entirely. Let me know your preference.

[fmeum]

@aiuto Sorry to bother you with this, but is there a way to sign the Google CLA for a single GitHub account with two Google accounts?

My last name changed and I now maintain two distinct Google accounts linked to my old and new name. I have signed the CLA with the old one and now can't add my new email to it because myaccount.google.com/alternateemail doesn't allow entering the address of another Google account. Do you see a way to unblock my commits under the new name?

[fmeum]

@sluongng Seems rather difficult to do, so please go ahead and squash away my author info.