[msbuild] No FrameworkList for XM Full other than 4.5, IDE reports missing assemblies

[VincentDondain]

Similar to #5367

Steps to Reproduce

1. Create a new XM project
2. Go to Project Options > Build > General > Xamarin.Mac Full
3. Select anything other than .NET 4.5

Expected Behavior

Assemblies in References should all be found.

Actual Behavior

Environment

=== Visual Studio Enterprise 2019 Preview for Mac ===

Version 8.0 Preview (8.0 build 2530)
Installation UUID: 00d602ef-271b-479b-9054-0433b5797215
	GTK+ 2.24.23 (Raleigh theme)
	Xamarin.Mac 5.6.0.2 (d16-0 / 040682909)

	Package version: 518000266

=== Mono Framework MDK ===

Runtime:
	Mono 5.18.0.266 (2018-08/29eb176dd97) (64-bit)
	Package version: 518000266

=== NuGet ===

Version: 4.8.0.5385

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	2.1.6
	2.1.2
	2.1.1
	2.0.5
	2.0.0-preview2-25407-01
	1.1.2
	1.0.5
SDK: /usr/local/share/dotnet/sdk/2.1.500/Sdks
SDK Versions:
	2.1.500
	2.1.302
	2.1.301
	2.1.4
	2.0.0-preview2-006497
	1.0.4
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.18.0/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.6.9
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Updater ===

Version: 11

=== Apple Developer Tools ===

Xcode 10.1 (14460.46)
Build 10B61

=== Xamarin.Mac ===

Version: 5.7.0.156 (Visual Studio Enterprise)
Hash: a03a0dfa3
Branch: fix-vsts-801867
Build date: 2019-02-26 18:25:38-0500

=== Xamarin.iOS ===

Version: 12.7.0.156 (Visual Studio Enterprise)
Hash: a03a0dfa3
Branch: fix-vsts-801867
Build date: 2019-02-26 18:25:38-0500

=== Xamarin Designer ===

Version: 4.17.4.366
Hash: b4b7c658d
Branch: remotes/origin/d16-0
Build date: 2019-02-18 13:35:26 UTC

=== Xamarin.Android ===

Version: 9.2.0.2 (Visual Studio Enterprise)
Android SDK: /Users/vidondai/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		4.4    (API level 19)
		4.4.87 (API level 20)
		5.0    (API level 21)
		5.1    (API level 22)
		6.0    (API level 23)
		7.0    (API level 24)
		7.1    (API level 25)
		8.0    (API level 26)
		8.1    (API level 27)

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 28.0.0
SDK Build Tools Version: 27.0.3

Build Information: 
Mono: mono/mono/2018-08@163f45d81ce
Java.Interop: xamarin/java.interop/d16-0@c987483
LibZipSharp: grendello/LibZipSharp/master@44de300
LibZip: nih-at/libzip/rel-1-5-1@b95cf3f
MXE: xamarin/mxe/xamarin@b9cbb535
ProGuard: xamarin/proguard/master@905836d
SQLite: xamarin/sqlite/3.26.0@325e91a
Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-0@0a7edd6

=== Microsoft Mobile OpenJDK ===

Java SDK: /Users/vidondai/Library/Developer/Xamarin/jdk/microsoft_dist_openjdk_8.0.25
1.8.0-25
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Android Device Manager ===

Version: 1.2.0.13
Hash: ec487ee
Branch: remotes/origin/d16-0
Build date: 2019-02-26 19:19:32 UTC

=== Xamarin Inspector ===

Version: 1.4.3
Hash: db27525
Branch: 1.4-release
Build date: Mon, 09 Jul 2018 21:20:18 GMT
Client compatibility: 1

=== Build Information ===

Release ID: 800002530
Git revision: b9bd60e9b9a0861393879c90863936e9c75c668a
Build date: 2019-02-27 14:00:07+00
Build branch: master
Xamarin extensions: af4010290a7e265c49e7d14d2684d8e8e0d984b9

=== Operating System ===

Mac OS X 10.14.3
Darwin 18.2.0 Darwin Kernel Version 18.2.0
    Thu Dec 20 20:46:53 PST 2018
    root:xnu-4903.241.1~1/RELEASE_X86_64 x86_64

Build Logs

Example Project (If Possible)

https://www.dropbox.com/s/zsrn2ickid4o7p6/SystemMemoryPain.zip?dl=0

[rolfbjarne]

This is not relevant in .NET anymore.