allow-newer should be suggested more often.

When trying to switch from LTS to nightly, with this in my `stack.yaml` (among other unimportant things),

```
extra-deps:
- map-syntax-0.3@sha256:84dc86fa1c292af25963bf7212ae7d55ce87239a9f8d4cc85bd0acc35874d2e1
```

I got this:

```
Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for map-syntax-0.3:
base-4.13.0.0 from stack configuration does not match >=4.3 && <4.13 (latest matching version is 4.12.0.0)
needed due to test3-0.0.0 -> map-syntax-0.3

In the dependencies for test3-0.0.0:
chronos needed, but the stack configuration has no specified version (latest matching version is 1.0.9)
string-interpolate needed, but the stack configuration has no specified version (latest matching version
is 0.1.0.1)
needed since test3 is a build target.

Some different approaches to resolving this:

* Build requires unattainable version of base. Since base is a part of GHC, you most likely need to use a
different GHC version with the matching base.

Plan construction failed.
```

The `base` bound is conservative, and the package compiles fine with 4.13. So by using `allow-newer: true` and adding the missing (from nightly) packages to `extra-deps` I was able to build without difficulty.

However, the advice does not suggest this, instead signalling hopelessness. If I understand the Stack code, `extra-deps` is not suggested if `base` is one of the problematic deps, and `allow-newer` is only suggested if it would fix _all_ the problems, which it won't because some `extra-deps` are needed. This creates a recommendation stalemate when in fact the solution was simple.

So I'm thinking `allow-newer` can be suggested even if it would not solve all problems. Or, perhaps it can be mentioned as an option in that "unattainable base" warning.

(Tangentially, it might be nice to be able to specify `allow-newer` on a per-dependency basis, but maybe that would be too troublesome.)