Additional refactoring of ManagedDependencies

[mattt]

As suggested in #3698 (comment), this PR provides additional refactoring of ManagedDependencies by rebasing changes in #3640 against #3698.

Motivation:

Improve convenience and correctness of Workspace APIs.

Modifications:

• Converted ManagedDependency from class to structure and made ManagedDependency.State an indirect enumeration. It makes sense for this to be a value type and has the added benefit of automatic Equatable synthesis.
• Removed ManagedDependency.packageIdentity property. This was a helper for self.packageRef.identity and only used in a couple places.
• Consolidated dependency path determination into dedicated APIs. There are a few places where we determine the location of a repository checkout or edited package by appending to a location. To ensure consistent routing (i.e. case normalization), these now go through a single API.
• Removed ManagedDependency.subpath property. The location of a managed dependency is determined by the workspace (except in the case of an unmanaged edited dependency, which stores its path separately), so we don't need to store it separately. I wasn't able to get this working. To be considered for a future refactoring pass.

[tomerd]

@matt #3698 should be ready for you to test against, when you are happy with it I will merge it as the rest of our tests pass

[mattt]

@swift-ci Please smoke test

[tomerd]

nice. thank you