Interpolate `StructUtils` into expressions

[jakobjpeters]

This will prevent errors like so:

julia> using StructUtils: @kwarg

julia> @kwarg struct X end
ERROR: UndefVarError: `StructUtils` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: StructUtils is loaded but not imported in the active module Main.
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

julia> using StructUtils: StructUtils

julia> @kwarg struct X end