Fix project relative imports

[sergv]

Fix bug where cabal tries to resolve imports by relative paths against the directory cabal executable was invoked in rather than directory of cabal.project file that does the import.

The problem occurs if <toplevel>/cabal.project does something like import: dep/cabal.project and then cabal build is invoked from some subridectory e.g. <toplevel>/dep2. Cabal tries to resolve dep/cabal.project against <toplevel>/dep2 but it probably should do the resolution against the <toplevel> directory that contains cabal.project that does the import.

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[gbaz]

Looks good, and like a correct fix. Please fix the warnings on the tests so CI passes :-)