The dead-code elimination (DCE) code has two filter strings and dependencies as a set of strings. The strings have special meanings such as both empty strings indicate always alive. To help facilitate future changes to DCE, I'm moving the DCE code to it's own package. Also I'm making some methods to set a decl's DCE filters and deps to ensure that the names are continued to be set the same as new changes are made.

These changes should have only minor functional changes, everything should continue to function as before. These changes are mostly a minor refactor to encapsulate DCE design decisions. The CI should continue to pass.

This is related to #1013 and #1270 work since the future changes to DCE is to include removal of dead generic code instantiations, e.g. allow List[int] to be alive and List[string] be dead separately.