[buildozer] unusedLoads removes loads of structs

If you run `buildozer 'fix unusedLoads|//src/foo:__pkg__'` on a file that loads a `struct` from some `.bzl` file and is actively using it, I'm observing that buildozer erroneously removes the load.

I'm not sure if there's a test for this or not, or if I'm simply using the feature wrong. Please let me know if I'm mistaken!

Reproduce the reported `buildozer 'fix unusedLoads|//src/foo:__pkg__'` behavior with a file that loads and uses a struct from a `.bzl` file. Find the `unusedLoads` implementation and any related tests; done means an actively used struct load is retained, with regression coverage if the existing tests support it.

Written by the indexing model from the issue text.