fake-package + --package-db=relative/path fails

[fgaz]

Describe the bug

fake-package + --package-db=relative/path fails

To Reproduce

$ cabal v2-repl -b some-package-in-db --package-db=relative/path/to/db
Resolving dependencies...
Build profile: -w ghc-9.0.2 -O1
In order, the following will be built (use -v for more details):
 - fake-package-0 (lib) (first run)
Warning: this is a debug build of cabal-install with assertions enabled.
Configuring library for fake-package-0..
Error: cabal: The following package dependencies were requested
--dependency='some-package-in-db=some-package-in-db-0.1.0.0'
however the given installed package instance does not exist.

Expected behavior

Should not fail. I suspect the path gets interpreted relatively to the fake-package temporary directory. It should be made absolute before using it that way.

System information

• NixOS 22.05
• ghc 9.0.2, cabal 8.9-something

Additional context

cc @patrickdoc (#7676)

Related to #6977