ghc-pkg update should fail if dependent packages might break

If a package is recompiled and re-installed, and other packages depend on it, those packages might be broken; e.g. see #3060 (closed).

Hence ghc-pkg update should refuse to perform the update, unless explicitly overridden with --force. However, it's not quite as simple as that:

• The package might already have been installed over the top of the old one, and

  it's too late to reverse that.

• cabal-install likes to build and install a bunch of packages at the same time,

  using ghc-pkg update on each one. This will fail.

• The GHC build system uses ghc-pkg update to register packages, and this

  breaks when packages are re-built (it's ok for a clean build, though).

I've attached a couple of patches to implement it, but let's hold these back until the above issues are sorted out.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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