x/tools/gopls: feature: integrate "spaghetti" dependency analysis

[adonovan]

https://github.com/adonovan/spaghetti is a static analysis tool for Go that helps you find out why your application depends on a given package, and lets you visualize what your dependencies would be if a given set of imports were eliminated. This can help you understand what work is required to break a dependency on a package that is undesirable, for instance because it is insecure, buggy, unmaintained or legally problematic.

Using its new web server, Gopls could integrate this tool to make it easier to find and invoke. For example, a Code Action associated with the go.mod file could invoke the tool to report the dependencies of that module, and a Code Action associated with a package declaration could invoke the tool to show how the main module depends on that package.