Nogo: analyzers flags are not passed to subanalyzers

[grzegorzsn]

Currently, nogo passes configuration only to top analyzers, skipping the required subanalyzers.

Why is it a a problem? Some analyzers split the configuration among a few subanalyzers. Good
example here is nilaway, which documentation explains how to pass some configuration options
to dedicated nilaway_config subanalyzer.

https://github.com/uber-go/nilaway/wiki/Configuration#bazelnogo

Example configuration looks like this:

{
  "nilaway_config": {
    "analyzer_flags": {
      "include-pkgs": "go.uber.org",
      "exclude-pkgs": "vendor/",
      "exclude-file-docstrings": "@generated,Code generated by,Autogenerated by"
    }
  },
  "nilaway": {
    "exclude_files": {
      "bazel-out": "this prevents nilaway from outputting diagnostics on intermediate test files"
    },
    "only_files": {
      "my/code/path": "This is the comment for why we want to enable NilAway on this code path"
    }
  },
}

However, as nilaway_config is just subanalyzer, the analyzer flags are never actually set by nogo.

Fix

This commit contains relevant fix. Please let me know if should file pull request in current form or
what adjustments are expected. I'd also love some guidance wrt. testing if needed.
d9fb014

[fmeum]

The commit looks good, please send a PR!

Re testing: Could you extend https://github.com/bazel-contrib/rules_go/blob/master/tests/core/nogo/custom/flags/flags_test.go with a new test case?

[grzegorzsn]

Thanks for pointing relevant tests.
Please find PR here: #4330