Change sourcekitd_* functions to public

[norio-nomura]

Fix #487

[norio-nomura]

Closing since #487 was closed.
I think that it is not good that there is no function to create sourcekitd_object_t which Request.customRequest needs.

[ileitch]

Oh, I thought this change was still valid. For example you can create a sourcekitd_object_t from sourcekitd_request_dictionary_create, e.g Request.customRequest(request: sourcekitd_request_dictionary_create(&keys, &values, dict.count)!).send().

Without this PR it makes little sense for customRequest to be public, I don't see any other way to use it?

[norio-nomura]

Sorry, reopened.

[norio-nomura]

As a personal opinion, I would like to recommend #490 than this.

[jpsim]

Agreed, let's pursue #490 over this. Thanks for leading the charge, @norio-nomura.