Add support for opting out of package environments

This implements the first part proposed in #13753:

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, a

- `-package-env -` CLI flag, or a
- `GHC_ENVIRONMENT=-` env var (unless a `-package-env` flag is present)

would inhibit GHC from looking up and interpreting any package
environment files from the filesystem; this is the equivalent of
`-ignore-dot-ghci` for package environment files.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #13753

Differential Revision: https://phabricator.haskell.org/D4690