Parameterize output package name

[ignasi35]

Current sbt plugin will output code into a package equal to the organization name of the sbt project. This value should be overridable by the end user.

See

sbt-lagom-descriptor-generator/lagom-descriptor-generator-sbt-plugin/src/main/scala/com/lightbend/lagom/spec/sbt/LagomOpenApiGenerator.scala

Line 36 in 070a71d

val packageName = organization.value

[ignasi35]

This is closely related to #25 since a user consuming several service may want to create their APIs in separate packages. Imagine an app consuming twitter, slack and G+ APIs and all three have an Error type in their definitions, without a package-per-service there would be a collision and client code would be confusing.

[ignasi35]

This could be achieved by either including a lagomGeneratorPackageName parameter in the sbt plugin or via allowing users to implement a Service => Service function in their sbt settings and have that injected in the provided generators.