Differentiate between declared (XSD) and enforcement (java.util.regex) patterns

The strings we are using for IllegalArgumentExceptions in generated code are Java Pattern regexes and are an implementation detail of how enforcement is done. End users should not be exposed to this detail.

yang-model-api is exposing both Java Pattern and XSD regex as declared in the model, hence codegen can capture the XSD regex and report that, leading to proper isolation between API contract (enforce the XSD regex (non-) match) and implementation (checking using Pattern).