restore support for -package-db

[gelisam]

Fixes #142

[gelisam]

The tests pass with cabal, but not with stack. Both use ghc-pkg under the hood though, so I don't understand why a stack-run test would not be able to load a package database created by cabal.

[gelisam]

Oh no! In the process of figuring out the difference between cabal and stack, I switched to ghc-8.10.7, and with that version the test is failing with cabal as well!

[gelisam]

The test passes with:

• cabal, ghc-9.2.1
• cabal, ghc-8.8.4

But fails with:

• cabal, ghc-8.10.7
• stack, ghc-9.2.1
• stack, ghc-8.10.7

The combination "stack, ghc-8.8.4" does not appear in either list because I can't even get to stack test, as stack build already fails with Prelude.chr: bad argument: 2600468483.

So I think there are two separate problems:

1. the test fails with stack
2. the test fails with ghc-8.10.7

[gelisam]

In both cabal repl and stack repl, I can successfully load a cabal-built package database:

λ> unsafeRunInterpreterWithArgs ["-package-db=/home/gelisam/working/haskell/hint/my-package/dist-newstyle/packagedb/ghc-9.2.1"] (setImports ["MyModule"])
Right ()

So what's different between the running the test suite and running the repl?

[gelisam]

Aha! stack test sets GHC_ENVIRONMENT to some file, while stack repl sets it to "-". That turned out to be the key difference, as setting it to "-" allows stack test to load the package-db. Unsetting it works too.

[gelisam]

More precisely, that file contains:

clear-package-db
global-package-db
package-db /home/gelisam/.stack/snapshots/x86_64-linux/aa6a7b0bd3b2a5e23297306cc07c915ab5a27aaabfa2af46eda508fef75733ab/9.2.1/pkgdb
package-db /home/gelisam/working/haskell/hint/.stack-work/install/x86_64-linux/aa6a7b0bd3b2a5e23297306cc07c915ab5a27aaabfa2af46eda508fef75733ab/9.2.1/pkgdb
package-id hint-0.9.0.5-6W0hq4jSyuP2V4U8FIOqob
package-id template-haskell-2.18.0.0
package-id HUnit-1.6.2.0-6u2bDpVgA3a6qfMIwFsdBJ
package-id base-4.16.0.0
package-id bytestring-0.11.1.0
package-id containers-0.6.5.1
package-id directory-1.3.6.2
package-id exceptions-0.10.4
package-id filepath-1.4.2.1
package-id ghc-9.2.1
package-id ghc-boot-9.2.1
package-id ghc-paths-0.1.0.12-FlEU4hxbdpqIe5aj4NDoD0
package-id random-1.2.1-18ubVgOsD0ELHvvhJVTjcY
package-id stm-2.5.0.0
package-id temporary-1.3-7JBHaoFPoLg44nCSFR89ij
package-id text-1.2.5.0
package-id transformers-0.5.6.2
package-id typed-process-0.2.7.0-KFeUVnIhBQ45qsp5kIfkdT
package-id unix-2.7.2.2

There are two reasons why this file is causing problems. First, it clears the list of package databases, so if this file happens to be interpreted after the command-line -package-db argument, then that argument has no effect. Second, it limits the list of packages which can be loaded to a list which does not include the my-package package which we're tying to load.

[gelisam]

Finally!