Make Package Manager Getters Const
Make Package Manager Getters Const

- Make methods GetStructureDefinition, GetSearchParameter, GetCodeSystem and
  GetValueSet on the FhirPackageManager class const.

PiperOrigin-RevId: 488793371

google/fhir,google/fhir,google/fhir,google/fhir,google/fhir,google/fhir