Allow ignoring global package db

For sandboxing, users want to be able to use only local package dbs. To handle rts, base etc, they copy those specific packages from the global package db into a local package db.

Currently however there is no sensible way to tell ghc to look only at the listed package dbs, and not the global db. There is a flag -no-user-package-conf to avoid looking at the per-user one, but no equivalent to ignore or to specify the global one.

(Note: as a quirky non-sensible way, you can use GHC_PACKAGE_PATH="" to clear the package db stack, but this should be possible using command line flags.)

The user interface we're now using in cabal is like this:

• --global sets the initial db stack to [GlobalPackageDB]
• --user sets the initial db stack to [GlobalPackageDB, UserPackageDB]
• --package-db=clear clears the db stack to []
• --package-db=global pushes GlobalPackageDB on the top of the stack
• --package-db=user pushes UserPackageDB on the top of the stack
• --package-db=${otherfile} pushes SpecificPackageDB ${otherfile} on the top of the stack

It seems to me we could do something similar with ghc. Currently in ghc we have a -hide-all-packages flag that resets the package set to empty. We could do with an equivalent for the package database stack. I suggest something like:

• -clear-package-conf
• -global-package-conf
• -user-package-conf (and the existing -no-user-package-conf)
• -package-conf file

The default would remain that the global and user dbs are used, but this can be reset using -clear-package-conf.

Personally I prefer the nomenclarture "package db" rather than "package conf" here, but it's not that important.

Trac metadata

Trac field	Value
Version	7.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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