Fix F# targets on xbuild 12.0 & 14.0

[mhutch]

On xbuild the MSBuildExtensionsPath property is multi-valued. However, this only works in Imports. When used in a property, it has a single value - which is not where the F# targets are installed on Mac.

As as result, the Xamarin targets were falling back to checking MSBuildBinPath, however the F# targets were only installed into the bin path of the 4.0 toolset, and so this failed with the 12.0 and 14.0 toolsets.

Also, make the locations we check for F# consistent across all the targets (maybe it should be factored out).

[dnfclas]

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[mhutch]

(see also dotnet/android#141)

[xamarin-release-manager]

Build failure

[spouliot]

@jstedfast can you review and merge (this one and the C8 based one) if everything is fine ? thanks!

[VincentDondain]

That looks good to me 👍

[spouliot]

@mhutch we have failures now on xcode8 for watchOS

/Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/fsharplibrary/fsharplibrary-watchos.fsproj: error : /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/fsharplibrary/fsharplibrary-watchos.fsproj: /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/_ios-build/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/WatchOS/Xamarin.WatchOS.FSharp.targets: Project file could not be imported, it was being imported by /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/fsharplibrary/fsharplibrary-watchos.fsproj: The required attribute "Project" in Import is empty

most project files (for newer platforms) are generated from the "basic" classic project - so it might be that some change needs to be reflected into the tool that convert the projects

[mhutch]

@spouliot the xcode8 branch does not have this commit. Maybe this needs to be backported...