Add whole-package deprecation warnings

With GHC-6.12 if you use base 3 you get:

    Warning: Module `Prelude' is deprecated:
                 [You are using an old version of base. Future
GHC versions will not support this version, so you need to
update your code to use the new base version.]

This is initially a somewhat confusing warning. We're not really deprecating the Prelude!

It would be much nicer if it told us:

    Warning: Package `base-3.0.3.2' is deprecated:
                 [You are using an old version of base. Future
GHC versions will not support this version, so you need to
update your code to use the new base version.]

The message itself could be formatted more nicely too. Patch attached for that.

Note the current spurious list syntax in the deprecation message. I've re-opened #3303 (closed) about that.

As for the syntax for package deprecations, how about just

module Foo {-# DEPRECATED package "the message" #-} (..) where

For the behaviour, how about making them behave just like module-deprecation messages in that one attaches them to a module header and they are only triggered if you import that module. The only change would be in how the message is reported; instead of saying the module is deprecated it'd give the package (including the full version).

I know this is a feature request, which usually we'd want to put off for the next major version, but since this one is all about helping maintainers and users of 6.12.1 and .2, I hope that we consider looking at it for an early 6.12 release.

Trac metadata

Trac field	Value
Version	6.12.1 RC1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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