Allow duplicate analyzer flags in nogo configuration

Our nogo config looks like this:
```
nogo(
name = "lint",
"importas": {
"analyzer_flags": {
# TODO: figure out how to pass more than one alias flag here :-/
"alias": "foo.com/package/errors:fooerr",
},
},
},
...
)
```

The [mportas](https://github.com/julz/importas) linter accepts multiple `-alias` flags to enforce different import alias mappings, but there does't appear to be a way to specify that in nogo.

importas usage:
```
importas \
-alias knative.dev/serving/pkg/apis/autoscaling/v1alpha1:autoscalingv1alpha1 \
-alias knative.dev/serving/pkg/apis/serving/v1:servingv1 \
./...
```

Start by tracing how nogo's analyzer_flags configuration is translated into command-line arguments, using the shown importas invocation as the expected behavior. Determine how multiple alias mappings can be represented, and consider the work complete when a nogo configuration can pass multiple -alias flags to importas.

Written by the indexing model from the issue text.