Create separate Top-level directory per platform

- Replace src\powershell top level project by two
  new root-level projects:
    - src\powershell-unix
    - src\powershell-windows
- Break src\Modules into more granular structure:
  - Shared
  - Windows+Unix-Core
  - Windows-Core
  - Windows-Core+Full
  - Windows-Full
  - To be created: Unix-Core. We will do it as a separate PR
- Fix #1122 : Platform-specific set of Modules (and assemblies)
- Fix Start-TypeGen to work with new top level folders layout

build.psm1

@@ -346,13 +346,6 @@ function New-PSOptions {
     # Add .NET CLI tools to PATH
     Find-Dotnet
 
-    if ($FullCLR) {
-        $Top = "$PSScriptRoot/src/Microsoft.PowerShell.ConsoleHost"
-    } else {
-        $Top = "$PSScriptRoot/src/powershell"
-    }
-    Write-Verbose "Top project directory is $Top"
-
     if (-not $Configuration) {
         $Configuration = if ($IsLinux -or $IsOSX) {
             "Linux"
@@ -362,6 +355,21 @@ function New-PSOptions {
         Write-Verbose "Using configuration '$Configuration'"
     }
 
+    if ($FullCLR) {
+        $Top = "$PSScriptRoot/src/Microsoft.PowerShell.ConsoleHost"
+    } else {
+        if ($Configuration -eq 'Linux')
+        {
+            $Top = "$PSScriptRoot/src/powershell-unix"
+        }
+        else
+        {
+            $Top = "$PSScriptRoot/src/powershell-windows"
+        }
+    }
+    Write-Verbose "Top project directory is $Top"
+
+
     if (-not $Framework) {
         $Framework = if ($FullCLR) {
             "net451"
@@ -1072,7 +1080,8 @@ function Start-TypeGen
     Push-Location "$PSScriptRoot/src/TypeCatalogParser"
     try
     {
-        dotnet run
+        $topPath = if ($IsWindows) {'../powershell-windows'} else {'../powershell-unix'}
+        dotnet run $topPath
     }
     finally
     {