stack build --pedantic shouldn't apply to packages marked extra-dep

[mboes]

Create a new project with stack new blah simple. Then unpack any odd package from Hackage with compiler warnings into vendor/foo, and make your project depend on it:

resolver: lts-3.11

packages:
- '.'
- location: vendor/foo
  extra-dep: true

Now compile everything with stack build --pedantic. The build will fail due to a compiler warning in foo. I would argue that since foo is a vendor package that the programmer has little control over, --pedantic shouldn't apply to any package marked with extra-dep: true.

[borsboom]

#1265 will have this effect, since extra-deps will be treated like snapshot packages and won't have command-line GHC flags applied.