Variables from xcconfig are not resolved when using $VAR syntax

This issue is somewhat related to https://github.com/CocoaPods/Xcodeproj/issues/505.

When defining build settings using variables from `xcconfig` files the `$VAR` and `$(VAR)` syntax work differently.

- Build setting defined as `$(VAR)` will be properly resolved to the value of `VAR`.
- Build setting defined as `$VAR` will be resolved into just `"$VAR"` string as is.

Start by locating the xcconfig build-setting resolution entry point and trace how `$VAR` and `$(VAR)` are handled. Compare both syntax paths and add coverage where the project tests variable resolution; done means the two forms resolve to the same VAR value.

Written by the indexing model from the issue text.