Separate API and client into its own modules

[pablochacin]

Presently the whole k6build module must be imported in order to use the client.

This creates transitive dependencies to all the dependencies needed by the k6build server to other components using the client (for example, k6provider) that will have to update the k6build module even when they are not using any of these dependencies.

Separating the client (and API) into their own modules will minimize the dependencies.

The main disadvantage is managing the release of the modules from one repository.