make graveyard an implicit dependency for nimble projects, to avoid breakages

[timotheecour]

proposal

• graveyard becomes an implicit dependency for nimble projects
• a warning (that can be turned off) is shown when something in graveyard is used

motivation

• this PR Move four modules to graveyard #10248 broke some projects (eg `nimongo + others that depend on it, see also Move four modules to graveyard #10248 (comment)); with this proposal, nothing would break, but a warning would be emitted when you start depending on modules that were moved to graveyard so you are reminded to use a better alternative
• this would make it easier to seamlessly evolve/cleanup stdlib without breaking "the world"

alternative

require users to call:

nimble install foo

for a pkg foo that was moved to graveyard

[Araq]

That we moved oids and smtp to the graveyard was a mistake, we're reverting this change. Graveyard is for abandoned modules.

[timotheecour]

closing, satisifed w the revert of oids, smtp

EDIT: but see #10248 (comment) which shows it's still controversial whether we shd keep oids or not, for example

[alehander92]

graveyard shouldn't be an implicit dependency, I want to see what I depend on in my nimble files and people should take an explicit decision if they want to continue using those modules