Improve GHC's ghc package environment lookup logic

Documentation for package enviroments which describes the current lookup logic: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/packages.html#package-environments

There's two things I'd like to improve:

1. Define a special magic "null" environment, which instructs GHC to ignore any package environment files.

To this end, I propose to use the name - (i.e. a single dash), as that is more portable than using the empty string for environment variables. In other words:

• -package-env - or
• GHC_ENVIRONMENT=- (unless a -package-env flag which has higher priority overrides it)

would prevent GHC from interpreting any package environment files.

1. Provide a way to disambiguate environment names (i.e. those that are looked up in $HOME/.ghc/arch-os-version/environments/name) and environment files arguments.

Currently, if you say GHC_ENVIRONMENT=test then a local file test would shadow the environment named test which would result in rather confusing errors. Instead I suggest to have package-env identifiers be interpreted as "names" by default, and only if they look like relative or absolute paths (i.e. ./foobar or /home/foo/...) be interpreted as file locations (relative to $CWD).

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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