After creating second executable, stack ghci no longer able to load library modules

### General summary/comments

After adding a second executable, `stack ghci` prompts me to choose which executable I want to load (or none). Regardless of my choice, the REPL is no longer able to load modules from my library.

Meta: I haven't made many second executables in the same project before, so possibly I messed something up there? It seems pretty straightforward though and felt like a bug.

### Steps to reproduce

Note: This repo has a reproduction of the issue: https://github.com/MaxGabriel/StackGhciIssue. The repo is generated from `stack new stack-issue haskeleton`, which is pretty barebones.

Steps:

1. Create a new stack project with an executable: `stack new stack-issue haskeleton`
2. `cd stack-issue`
3. Run `stack ghci` to confirm it works.
4. Add a new executable to the list like so (copied from the first executable to be identical besides the executable name and main module name):
```
second-executable:
dependencies:
- base
- stack-issue
ghc-options:
- -rtsopts
- -threaded
- -with-rtsopts=-N
main: Main2.hs
source-dirs: executable
```
5. Add a `Main2.hs` file to the executables directory. Copy the contents of `Main.hs` into it.
6. Run `stack ghci`

### Expected

`stack ghci` worked, and allowed me to import modules from my library.

### Actual

`stack ghci` asked me which executable to use, or none. Every option chosen is unable to import modules from the library. In the repo reproducing the issue, I get the error message:

```
Specify main module to use (press enter to load none): 1
Loading main module from candidate 1, --main-is /Users/maximiliantagher/Documents/tmp/stack-issue/executable/Main2.hs

GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help

/Users/maximiliantagher/Documents/tmp/stack-issue/executable/Main2.hs:1:1: error:
File name does not match module name:
Saw: ‘Main’
Expected: ‘Main2’
Failed, modules loaded: none.

/Users/maximiliantagher/Documents/tmp/stack-issue/executable/Main2.hs:1:1: error:
File name does not match module name:
Saw: ‘Main’
Expected: ‘Main2’
Failed, modules loaded: none.

: error:
Could not find module ‘Example’
It is not a module in the current program, or in any known package.
Loaded GHCi configuration from /private/var/folders/v7/5k06hq7d3wbb4cvk8smv6kl00000gn/T/ghci92913/ghci-script
```

When I try this in a scaffolded Yesod repo, I get a similar error message, but my modules are listed as being part of a hidden package:

```
: error:
Could not find module ‘Application’
It is a member of the hidden package ‘mercury-web-backend-0.0.0@mercury-web-backend-0.0.0-3D8Wjw6Swy0FVZjLiDp4Vn’.
```

I'm able to workaround this issue in the Yesod repo by doing: `stack ghci packageName:executableName`, which lets me import modules from my Yesod app. I'm not able to get this workaround to work in the repo reproducing the issue, I'm not sure why.

Output of `stack ghci` in verbose mode, pressing enter to choose neither executable: https://gist.github.com/MaxGabriel/56e2efe78312ebc62aa4e7c319da16a2

### Stack version

```
stack --version
Version 1.5.1, Git revision 600c1f01435a10d127938709556c1682ecfd694e (4861 commits) x86_64 hpack-0.17.1
```

I also tested with:

```
stack1.6 --version
Version 1.6.0.20171022, Git revision 7bddfaf7f9f8cd9ec1c710fa83e77262e494eee4 (5285 commits) x86_64 hpack-0.18.1
```

### Method of installation

* Official binary