feat: use built-in Newtonsoft.Json if possible (#43)

If PowerShell ships with a version of Newtonsoft.Json, use it through an assembly binding.
If not, try to load the one we ship with the library.

This will reduce the likelihood of conflicts at assembly loading time, and makes the library
work under Powershell 6.0.x. Fixes #36.

Also, stop whenever there is an error importing the module, to make
debugging easier.

FeatureFlags.psm1

@@ -1,3 +1,4 @@
+$ErrorActionPreference = "Stop"
 <#
 .SYNOPSIS 
 Loads the feature flags configuration from a JSON file.
@@ -18,28 +19,38 @@ function Get-FeatureFlagConfigFromFile([string]$jsonConfigPath) {
 }
 
 # This library uses Test-Json for JSON schema validation for PowerShell >= 6.1.
-# For previous versions, it uses NJsonSchema, # which depends on Newtonsoft.JSON.
+# For previous versions, it uses NJsonSchema, which depends on Newtonsoft.JSON.
 # Since PowerShell itself uses NJsonSchema and Newtonsoft.JSON, we load these
 # assemblies only when it is needed (older PowerShell versions).
 $version = $PSVersionTable.PSVersion
 Write-Verbose "Running under PowerShell $version"
 if ($version -lt [System.Version]"6.1.0") {
     Write-Verbose "Loading JSON/JSON Schema libraries"
-    # Import the JSON and JSON schema libraries, and load the JSON schema.
-    $libs = Get-ChildItem -Recurse -Path "$PSScriptRoot/External"
-    $libs = $libs | Where-Object {$_.Extension -ieq ".dll" -and $_.FullName -ilike "*netstandard1.0*"} | ForEach-Object {$_.FullName}
-    $schemaLibPath = $libs | Where-Object {$_ -ilike "*NJsonSchema.dll"}
-    if (-not (Test-Path -Path $schemaLibPath -PathType Leaf)) {
-        Write-Error "Could not find the DLL for NJSonSchema: $schemaLibPath"
-    }
-    Write-Verbose "Found NJsonSchema assembly at $schemaLibPath"
 
-    $jsonLibPath = $libs | Where-Object {$_ -ilike "*Newtonsoft.Json.dll"}
-    if (-not (Test-Path -Path $jsonLibPath -PathType Leaf)) {
-        Write-Error "Could not find the DLL for Newtonsoft.Json: $jsonLibPath"
+    # Get DLLs imported via restore.
+    $externalLibs = Get-ChildItem -Recurse -Path "$PSScriptRoot/External"
+    $externalLibs = $externalLibs | Where-Object {$_.Extension -ieq ".dll" -and $_.FullName -ilike "*netstandard1.0*"} | ForEach-Object {$_.FullName}
+
+    # If PowerShell ships with Newtonsoft.JSON, let's load that copy rather than the one in the NuGet package.
+    $jsonLibPath = [System.AppDomain]::CurrentDomain.GetAssemblies() | Where-Object {$_.FullName.StartsWith("Newtonsoft.Json")} | Select-Object -ExpandProperty Location
+    if ($null -eq $jsonLibPath) {
+        $jsonLibPath = $externalLibs | Where-Object {$_ -ilike "*Newtonsoft.Json.dll"}
+
+        if (-not (Test-Path -Path $jsonLibPath -PathType Leaf)) {
+            Write-Error "Could not find the DLL for Newtonsoft.Json: $jsonLibPath"
+        }   
+
+        try {
+            $jsonType = Add-Type -Path $jsonLibPath -PassThru
+            Write-Verbose "JSON.Net type: $jsonType"
+        } catch {
+            Write-Error "Error loading Newtonsoft.Json libraries ($jsonLibPath): $($_.Exception.Message)"
+            throw
+        }   
     }
-    Write-Verbose "Found JSON.Net assembly at $jsonLibPath"
+    Write-Verbose "Using Newtonsoft.JSON from $jsonLibPath"
 
+    # Add an assembly redirect in case that NJsonSchema refers to a different version of Newtonsoft.Json.
     Write-Verbose "Adding assembly resolver."
     $onAssemblyResolve = [System.ResolveEventHandler] {
         param($sender, $e)
@@ -54,13 +65,18 @@ if ($version -lt [System.Version]"6.1.0") {
     }
     [System.AppDomain]::CurrentDomain.add_AssemblyResolve($onAssemblyResolve)
 
+    # Load the JSON Schema library.
+    $schemaLibPath = $externalLibs | Where-Object {$_ -ilike "*NJsonSchema.dll"}
+    if (-not (Test-Path -Path $schemaLibPath -PathType Leaf)) {
+        Write-Error "Could not find the DLL for NJSonSchema: $schemaLibPath"
+    }
+    Write-Verbose "Found NJsonSchema assembly at $schemaLibPath"
+
     try {
-        $jsonType = Add-Type -Path $jsonLibPath -PassThru
         $jsonSchemaType = Add-Type -Path $schemaLibPath -PassThru
-        #Write-Verbose "JSON.Net type: $jsonType"
-        #Write-Verbose "NjsonSchema type: $jsonSchemaType"
+        Write-Verbose "NjsonSchema type: $jsonSchemaType"
     } catch {
-        Write-Error "Error loading JSON libraries ($jsonLibPath, $schemaLibPath): $($_.Exception.Message)"
+        Write-Error "Error loading JSON schema library ($schemaLibPath): $($_.Exception.Message)"
         throw
     }
 
@@ -467,4 +483,4 @@ function Out-FeaturesEnvConfig
             }
         }
     }
-}
+}