[msbuild] Fix import set to expected path when invalid imports are allowed.

It was failing to resolve <Import> to correct path because it allowed
invalid imports too early in Import.ForEachExtensionPathTillFound().

It is due to Project.AddSingleImport() that checks if invalid imports
can be ignored by load setting. But simply removing this check results
in regressions in nunit tests that fails to report required invalid imports.

This bug was one of the blockers to build fsharp-droid from xbuild:
https://github.com/fahadsuhaib/fsharp-droid