Reject solver solutions involving un-buildable libraries.

[phadej]

Now you get something like this:

Resolving dependencies...
cabal: Could not resolve dependencies:
trying: p-0.1 (user goal)
unknown package: unbuildable (dependency of p-0.1)
fail (backjumping, conflict set: p, unbuildable)
Dependency tree exhaustively searched.

The error message is not the best.

Fixes #3978

---

% rm -rf dist-newstyle              
% cabal new-build -w ghc-7.8.4 all --dry
Resolving dependencies...
Error:
    Dependency on unbuildable library from http-client
    In the stanza 'library'
    In the package 'http-client-tls-0.3.5.3'

% rm -rf dist-newstyle                  
% ../build/cabal/cabal new-build -w ghc-7.8.4 all --dry
Resolving dependencies...
Build profile: -w ghc-7.8.4 -O1
In order, the following would be built (use -v for more details):
 - Cabal-2.0.1.1 (lib) (requires download & build)

...

% cabal-plan topo | grep http-client
http-client-tls-0.3.5.3
http-client-0.5.12

Note how solver figured out to pick http-client-0.5.12!

[phadej]

Actual output differs from expected:
stderr:
--- "C:\\projects\\cabal\\cabal-testsuite\\PackageTests\\NewBuild\\T3978\\cabal.dist\\cabal.out.normalized"	2018-05-19 12:27:09.841643700 +0000
+++ "C:\\projects\\cabal\\cabal-testsuite\\PackageTests\\NewBuild\\T3978\\cabal.dist\\cabal.comp.out.normalized"	2018-05-19 12:27:09.841643700 +0000
@@ -1,6 +1,7 @@
 # cabal new-build
 Resolving dependencies...
-Error:
-    Dependency on unbuildable library from p
-    In the stanza 'library'
-    In the inplace package 'q-1.0'
+cabal: Could not resolve dependencies:
+[__0] trying: p-1.0 (user goal)
+[__1] unknown package: buildable-false-hack (dependency of p)
+[__1] fail (backjumping, conflict set: buildable-false-hack, p)
+After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: p (2), buildable-false-hack (1)

[phadej]

Closing this in favour of @grayjay patch (btw, it's in use in servants CI, and it works!)