Improve error message for multiple matching packages for "stack script"

[mgsloan]

Current error message if you run stack script on a file with import Text.Mustache is

Module Text.Mustache appears in multiple packages: mustache stache
Called from:
  throwString (src/Stack/Script.hs:151:41 in stack-1.6.0-8X4wAceLUOM24tZpnp4UV1:Stack.Script)

Ideally this should give the following info:

• It should explain that it is figuring out packages to unhide based on parsing imports.
• The inferred --package arguments for the other packages. This way, if the user needs to switch to using --package arguments, they can just copy the ones that have been inferred and then add the disambiguating one.

[pbrinkmeier]

Hi, I use Stack's script feature a lot myself and I'd like to fix this as my first Stack issue.

I'd start by changing the error message in https://github.com/commercialhaskell/stack/blob/master/src/Stack/Script.hs#L58.
In order to do that, I'd change the exception type in https://github.com/commercialhaskell/stack/blob/master/src/Stack/Script.hs#L49 to something like:

AmbiguousModuleName ModuleName [PackageName] [PackageName]

where the newly introduced field contains the name of inferred packages.

Does this seem reasonable?