Typos in extra-deps aren't detected

[mgsloan]

Noticed this while helping L8D in #haskell

Repro:

1. stack new issue1521
2. cd issue1521
3. Add hspec >= 2.2.1 to the test-suite build-depends
4. Add the following to extra-deps:

- hspec-2.2.1 
- hspec-core-2.2.1 
- hspec-discover-2.2.1 
- hspec-epectations-0.7.2 

(Note the mis-spelled hspec-epectations)
5) stack test
6) Get this error:

While constructing the BuildPlan the following exceptions were encountered:

--  Failure when adding dependencies:    
      hspec-core: needed (==2.2.1), couldn't resolve its dependencies
      hspec-expectations: needed (==0.7.2.*), 0.7.1 found (latest applicable is 0.7.2)
    needed for package: hspec-2.2.1

--  Failure when adding dependencies:    
      hspec-expectations: needed (==0.7.2.*), 0.7.1 found (latest applicable is 0.7.2)
    needed for package: hspec-core-2.2.1

--  Failure when adding dependencies:    
      hspec: needed (>=2.2.1), couldn't resolve its dependencies
    needed for package: issue1521-0.1.0.0