Optionally make intermediate filler groups

This commit adds a new option `createIntermediateGroups` that defaults
to false. When it is false, the behavior of XcodeGen is the same as
before. When it is true, we make intermediate groups recursively until
we reach the basePath. In practice that means if you've chosen
`Platform/PINFoundation/Sources` as one of your sourcePaths, you get a
top-level group of `Platform` and under that `PINFoundation` and under
that `Sources`. This is instead of the default behavior of just making
`Sources` a top-level group (which is confusing when your directory is
called `Sources` for example).

Sources/XcodeGenKit/PBXProjGenerator.swift

@@ -512,6 +512,47 @@ public class PBXProjGenerator {
         return (fromFiles + fromDirs).flatMap { $0.sourceFiles }
     }
 
+    func getSingleGroup(path: Path, mergingChildren children: [String], depth: Int = 0) -> PBXGroup {
+        let group: PBXGroup
+        if let cachedGroup = groupsByPath[path] {
+            cachedGroup.children += children
+            group = cachedGroup
+        } else {
+            group = PBXGroup(
+                reference: generateUUID(PBXGroup.self, path.lastComponent),
+                children: children,
+                sourceTree: .group,
+                name: path.lastComponent,
+                path: depth == 0 && !spec.options.createIntermediateGroups ?
+                    path.byRemovingBase(path: spec.basePath).string :
+                    path.lastComponent
+            )
+            addObject(group)
+            groupsByPath[path] = group
+        }
+        return group
+    }
+
+    // Add groups for all parents recursively
+    // ex: path/foo/bar/baz/Hello.swift -> path:[foo:[bar:[baz:[Hello.swift]]]]
+    func getIntermediateGroups(path: Path, group: PBXGroup) -> PBXGroup {
+        // verify path is a subpath of spec.basePath
+        guard Path(components: zip(path.components, spec.basePath.components).map{ $0.0 }) == spec.basePath else {
+            return group
+        }
+
+        // base case
+        if path == spec.basePath {
+            return group
+        }
+
+        // recursive case
+        return getIntermediateGroups(
+            path: path.parent(),
+            group: getSingleGroup(path: path, mergingChildren: [group.reference])
+        )
+    }
+
     func getSources(path: Path, children: [Path]? = nil, depth: Int = 0) throws -> (sourceFiles: [SourceFile], groups: [PBXGroup]) {
         let children = try children ?? (try path.children())
         let excludedFiles: [String] = [".DS_Store"]
@@ -605,17 +646,18 @@ public class PBXProjGenerator {
             }
         }
 
-        let groupPath: String = depth == 0 ? path.byRemovingBase(path: spec.basePath).string : path.lastComponent
         let group: PBXGroup
-        if let cachedGroup = groupsByPath[path] {
-            group = cachedGroup
+        if spec.options.createIntermediateGroups {
+            group = getIntermediateGroups(
+                path: path.parent(),
+                group: getSingleGroup(path: path, mergingChildren: groupChildren, depth: depth)
+            )
         } else {
-            group = PBXGroup(reference: generateUUID(PBXGroup.self, path.lastComponent), children: groupChildren, sourceTree: .group, name: path.lastComponent, path: groupPath)
-            addObject(group)
-            if depth == 0 {
-                topLevelGroups.append(group)
-            }
-            groupsByPath[path] = group
+            group = getSingleGroup(path: path, mergingChildren: groupChildren, depth: depth)
+        }
+
+        if depth == 0 {
+            topLevelGroups.append(group)
         }
         groups.insert(group, at: 0)
         return (allSourceFiles, groups)