Calculating whether a .NET Standard library ref is a facade costs 0.5% of solution-wide design time build

[davkean]

See: dotnet/sdk#1586

Note: This is the most expensive condition in the entire build and give it only runs for .NET Standard projects it's overhead will be higher for a solution containing only .NET Standard projects.

Pass	File	Line #	Expression	Inc (ms)	Inc (%)	Exc (ms)	Exc (%)	#	Bug
Total Evaluation				6159	100%	54	0.9%
Properties (Pass 1)				3312	53.8%	9	0.1%
ItemDefinitionGroup (Pass 2)				14	0.2%	2	0%
Items (Pass 3)				566	9.2%	24	0.4%
Lazy Items (Pass 3.1)				1826	29.6%	158	2.6%
UsingTasks (Pass 4)				59	1%	59	1%
Targets (Pass 5)				327	5.3%	208	3.4%
Lazy Items (Pass 3.1)	NETStandard.Library.targets	12	Condition="'%(FileName)' != 'netstandard'")	33	0.5%	33	0.5%	43

This line: https://github.com/dotnet/standard/blob/master/netstandard/pkg/targets/NETStandard.Library.targets#L12

[ericstj]

Dup of #442 and already fixed by proposed solution: ericstj@1a06278