schema: add lysc_node_lref_xpath_targets() helper

[bradh352]

The lysc_node_lref_targets() helper returns a node in the schema, but if the xpath itself had added restrictions such as only matching certain list keys, then there was no way to retrieve that information.

This adds the appropriate helper, sharing as much code as possible with lysc_node_lref_targets() and also adds a test case to check for this.

Fixes #2357

[michalvasko]

As I mentioned in the linked issue, I would strongly prefer this helper function to be implemented in your project, has little use and hence redundant to be included in libyang.

[bradh352]

I'm closing this one. I ended up not using it directly in SONiC. Given the fact that the paths returned aren't valid xpaths, it is of minimal use. For Golang I just implemented this directly as cgo code, then rewrote the xpaths to be valid. For python a different approach is used there where it uses the original lysc_node_lref_targets() to look up the data paths that could be impacted, then scans all results doing a value check to limit to the actual matching nodes. A bit more overhead ... but the old libyang1 code was actually doing something similar so its no worse.