File watch doesn't trigger build when package.yaml changes

[tfausak]

If I edit my package.yaml (for hpack) while running stack build --file-watch, a new build doesn't start. Seems like it should.

[mitchellwrosen]

The list of watched files is populated with:

• all local packages' files
• stack.yaml

Perhaps Stack.Build.Source.getPackageFilesSimple should be made aware of package.yaml?

[mgsloan]

@tfausak Good catch, I think this commit fixes it - reopen if not.