Cabal should use the minimum version that satisfies the dependencies, not the maximum

[Brian Smith]

Let's say we have multiple versions of a package installed:
    a-1.0
    a-1.0.1
    a-1.1
    a-2.0
and that we have the following Build-Depends in a package description:
    Build-Depends: a >= 1.0

Now, when I configure, Cabal will choose a-2.0 as the package to build
with. However, it makes more sense to choose a-1.0 by default instead.

* Currently, if we change the program to require a feature that is in
a-2.0, but not in a-1.0, then the build will succeed for _us_, but it
will fail unexpectedly for someone else that has only a-1.0. However,
if Cabal instead chose the minimum version available, then, _our_
build would fail, and we would know that we need to either remove the
dependency on the a-2.0 feature or increase the version specified in
Build-depends.

* a-2.0 is not guarenteed to be 100% backwards-compatible with a-1.0.
Even a-1.0.1 is not guarenteed to be backwards-compatible. So, if the
original author was really testing with a-1.0, then we are more likely
to run into problems with a-2.0 (or even a-1.0.1) than a-1.0.

We might say that a-1.0.1 is probably a bugfix for a-1.0 that does not
change the API for the package. But, we can only guess about that;
maybe a-1.1 is a bugfix release too, but probably not. Furthermore, it
might be the case that the library/program in question is not affected
by the bugs in a-1.0, in which case it is okay to use a-1.0 even if
a-1.0.1 is available. And, if it is affected, then the Build-depends
should be changed to a >= 1.0.1 anyway.

Practically, a package author will often want his package to work with
the minimum versions of its dependencies AND the maximum versions at
the same time. To support this, we might want to add a flag to
"configure" that says "choose the maximum versions instead of the
minimum versions" which would result in the the current behavior. This
would allow the package author to build and test his package twice,
once with its minimum dependencies (by omitting the flag), and once
with the latest versions (by including the flag).

- Brian