LSP codegen: should probably use `sources` instead of `imports`

[Jakub Kozłowski]

As per https://smithy.io/2.0/guides/smithy-build-json.html#using-smithy-build-json:

imports - Provides a list of model files and directories to load when validating and building the model. Imports are a local dependency: they are not considered part of model package being built, but are required to build the model package.

sources - Provides a list of relative files or directories that contain the models that are considered the source models of the build.

We currently generate imports.

[Olivier Mélois]

well let's change it then.