[dotnet] use **FromWorkload** for targeting/runtime pack versions

Context: dotnet/sdk#19596
Context: dotnet/android#6184

If we use the version number string of `**FromWorkload**`, then our
runtime packages don't need to be resolved from a NuGet feed. They can
be resolved from the `dotnet/packs` directory.

This completely eliminates the need for a `NuGet.config` file when
building a .NET 6 app with a local build of the xamarin-macios.

You will no longer need a feed such as:

    <add key="local-dotnet-feed" value="~/src/xamarin-macios/_build/nuget-feed" />

dotnet/Makefile

@@ -167,8 +167,6 @@ nupkgs/$(1)$(4).$(2).nupkg: $(TEMPLATED_FILES) $(WORKLOAD_TARGETS) $(3) package/
 	$$(Q_PACK) $(DOTNET6) pack package/$(1)/package.csproj -p:VersionBand=$(DOTNET6_VERSION_BAND) --output "$$(dir $$@)" $(DOTNET_PACK_VERBOSITY) "/bl:$$@.binlog"
 	@# Clean the local feed
 	$$(Q_NUGET_DEL) if test -d $(DOTNET_FEED_DIR)/$(shell echo $(1) | tr A-Z a-z)/$(2); then nuget delete $(1) $(2) -source $(abspath $(DOTNET_FEED_DIR)) -NonInteractive $(NUGET_VERBOSITY); fi
-	@# Add the nupkg to our local feed
-	$$(Q_NUGET_ADD) nuget add "$$@" -source $(DOTNET_FEED_DIR) -NonInteractive $(NUGET_VERBOSITY)
 endef
 
 define CreateWindowsNuGetTemplate
@@ -363,6 +361,7 @@ export DOTNETSDK_WORKLOAD_MANIFEST_ROOTS:=$(abspath ./Workloads)
 	@# The microsoft.net.sdk.maui workload has a reference to the microsoft-ios-sdk-full workload, which has been renamed, so 'dotnet workload install' fails. Just remove the maui workload, we don't need it here.
 	$(Q) rm -Rf $(DOTNET6_DIR)/sdk-manifests/6.0.100/microsoft.net.sdk.maui
 	$(Q) cd Workloads && $(DOTNET6) workload install --skip-manifest-update \
+		--source $(abspath $(DOTNET_FEED_DIR)) --verbosity diag \
 		$(foreach platform,$(DOTNET_PLATFORMS),$(shell echo $(platform) | tr A-Z a-z))
 	$(Q) touch $@
 

dotnet/targets/Xamarin.Shared.Sdk.DefaultItems.targets

@@ -85,10 +85,10 @@
     <KnownFrameworkReference Include="Microsoft.$(_PlatformName)"
                             TargetFramework="net6.0"
                             RuntimeFrameworkName="Microsoft.$(_PlatformName)"
-                            DefaultRuntimeFrameworkVersion="$(_ShortPackageVersion)"
-                            LatestRuntimeFrameworkVersion="$(_ShortPackageVersion)"
+                            DefaultRuntimeFrameworkVersion="**FromWorkload**"
+                            LatestRuntimeFrameworkVersion="**FromWorkload**"
                             TargetingPackName="Microsoft.$(_PlatformName).Ref"
-                            TargetingPackVersion="$(_ShortPackageVersion)"
+                            TargetingPackVersion="**FromWorkload**"
                             RuntimePackNamePatterns="Microsoft.$(_PlatformName).Runtime.**RID**"
                             RuntimePackRuntimeIdentifiers="$(_RuntimePackRuntimeIdentifiers)"
                             Profile="$(_PlatformName)"

tests/dotnet/Makefile

@@ -6,7 +6,6 @@ include $(TOP)/Make.config
 # and to put any extracted packages in the 'packages' directory (to not clutter up ~/.nuget/packages)
 NuGet.config: $(TOP)/NuGet.config Makefile
 	$(Q) $(CP) $< $@.tmp
-	$(Q) nuget sources add -Name local-dotnet-feed -Source $(abspath $(DOTNET_FEED_DIR)) -ConfigFile $@.tmp
 ifdef CUSTOM_DOTNET
 	$(Q) nuget sources add -Name dev-runtime-feed -Source $(abspath $(DOTNET_RUNTIME_PATH)/artifacts/packages/Release/Shipping) -ConfigFile $@.tmp
 endif