Make importing easier with "Copy Import Directive" menu item.

[bwilkerson]

There is an outstanding request to support code completion of identifiers that are not yet imported into the current scope, but until / unless that happens it would be really convenient if there were a "Copy Import Directive" menu item (minimally on the file tab and in the Project view).

Similar to the way "Copy Path" works, it would compute the package: URI that should be used to import the selected file and put the text on the clipboard so that it could then be pasted into an editor.

[alexander-doroshko]

IntelliJ IDEA 2019.1 + recent Dart SDK versions do support completion of identifiers that are not yet imported. So probably this request could be closed.

[FMorschel]

@bwilkerson I think this and #30295 could probably be closed. I'm not sure about the mentioned issue but I take it asks for a similar thing from what I could gather.