Refactor identifier logic in `pkg/generate/code`

[brycahta]

Is your feature request related to a problem?
No, this is to consolidate and clean up some recently introduced code that works with shape and model identifiers.

Describe the solution you'd like

crd.go

• Move getSanitizedMemberPath and FindIdentifiersInCRD here as member functions
• Introduce GetPrimaryIdentifier member func. Precedence: Arn, Id, Name
  • Not seeing the value in this when FindPrimaryIdentifierFieldNames exists

check.go

• Move getSanitizedMemberPath out of here

common.go

• Move Find members relating to CRD to crd.go
• Add more unit tests to common_test.go

set_resource.go

• Refactor setResourceForScalar() to no longer construct paths, but simply take fully-qualified target var name and the source var name to set it to.
  • users of setResourceForScalar that use CRD members can use getSanitizedMemberPath to construct fully-qualified name
    • most users of that func also require targetShapeRef which getSanitized doesn't support right now.

set_sdk.go

• Refactor setSDKReadMany() to use refactored identifier logic

Describe alternatives you've considered
Discussed with @RedbackThomson offline

[brycahta]

All enhancements have been implemented; resolving.