Add completions for declaration name

When refactoring or copy/pasting code around it's very easy to paste in a fragment of code that points to non-existend identifiers in its new home.

It would be great if we could offer completions for variable, param, resource, and module names that include the non-existent identifiers.

No files, tests, or entry points are named. First clarify how completions should behave for unresolved variable, parameter, resource, and module identifiers, then locate the completion entry point and existing completion tests; done means these identifiers are offered without breaking current completion behavior.

Written by the indexing model from the issue text.