Static graph overbuilds when an inner build explicitly references another inner build

Created from https://github.com/Microsoft/msbuild/pull/4218#discussion_r267801871

Given two multitargeting projects A and B, where an inner build from A (e.g. A(net46)) explicitly references an innerbuild from B (e.g. B(net46)) by setting `Properties="TargetFramework=net46"` metadata on `ProjectReference`, the static graph overbuilds:
- in vanilla msbuild, A(net46) calls into B outer build, and into B(net46)
- in static graph, A(net46) references B outer build and all the inner builds of B

Failing test: https://github.com/Microsoft/msbuild/blob/55aab3b142b97ad5cf897ac3543af1ac6275808a/src/Build.UnitTests/Graph/ProjectGraph_Tests.cs#L1467-L1468