Input:
Request Id: 'source.request.editor.open.interface'
Required Field: 'key.modulename'
Optional Field: 'key.groupname'
Output:
If the group name is specified, SourceKit generates interface for the
module group; otherwise does for the entire module.
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.