Implemented by going through all packages in the 'depResolverSourcePkgIndex' and
applying 'relaxUpperBound' to the dependencies listed in 'build-depends'.

Known issue:
'build-depends: a < 3 && >= 2; if (someFlag): build-depends: a >= 5 && < 6'
gets converted to
'build-depends: >= 2; if (someFlag): build-depends: a >= 5'
(4 is now allowed where it previously wasn't).

Example:

    $ cabal install --dry-run ./tst
    Resolving dependencies...
    In order, the following would be installed (use -v for more details):
    array-0.3.0.3 (latest: 0.5.0.0)
    tst-0.1.0.0 (latest: 0.1.1)

    $ cabal install --dry-run --allow-newer=array ./tst
    Resolving dependencies...
    In order, the following would be installed (use -v for more details):
    tst-0.1.0.0 (latest: 0.1.1)