Add support for deprecating scoped SubsystemDependencies

[stuhood]

Problem

In order for a v2 Goal (in #6880) to replace a goal that was previously implemented as a collection of v1 Tasks, we need the ability to declare a deprecated dependency on a Subsystem (concretely: CacheSetup, in that case).

As explained in the patch, this is different from Optionable.deprecated_scope in one fundamental way: with deprecated_scope options live in two locations simultaneously (one of which is deprecated), while with a deprecated dependency they live in one location (which is deprecated). This split plays out in the unit tests, which verify that Optionable.deprecated_scope allows for splitting of a deprecated scope between two new destinations.

Solution

Add a removal_version to SubsystemDependency and ScopeInfo, and consume it in Options in order to render a warning for a deprecated ScopeInfo.

The ability to deprecate a scoped dependency is exposed via Subsystem.scoped(cls, removal_version=.., removal_hint=..).

Result

Callers can use Subsystem.scoped(cls, removal_version=.., removal_hint=..) to deprecate a scoped Subsystem dependency. #6880 will be unblocked to introduce a deprecated dependency on CacheSetup for v2 goals.

[illicitonion]

Looks reasonable.

When will this actually show up? When you run a goal which depends on a deprecated subsystem scope? Or only when you explicitly pass a flag from a deprecated subsystem scope?

[stuhood]

Or only when you explicitly pass a flag from a deprecated subsystem scope?

Only in this case.

[benjyw]

Sorry for the delayed review - was on a plane. This all LGTM.