Improve named mapping support in grails

At the moment navigation from gsp template to mapping definition works with clicking cmd+b on mapping name in `link` tag:
```

```
But it doesn't work in `redirect` in controller code:
```
redirect(mapping:'pluginСard', params: [pr: request.productId, headline: plugin.getUrlHeadline()])
```
Also Find Usages on mapping name in mapping definition (in UrlMapping.groovy file) doesn't work.

---

*Imported from [IDEA-164127](https://youtrack.jetbrains.com/issue/IDEA-164127) · Type: Feature · Votes: 0*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Start by tracing the existing Cmd+B navigation from the mapping name in the GSP `link` tag. Compare that path with the controller `redirect` example, then inspect `UrlMapping.groovy` and its mapping names for Find Usages behavior. Done means navigation works from `redirect` and Find Usages works from mapping definitions.

Written by the indexing model from the issue text.