allow FSharpTargetsPath to be properly overwritten in a project file

[brettfo]

Followup to #4922.

By previously setting FSharpTargetsPath, etc. directly we didn't allow for that to be manually set in a project, props, or target file. By using this method the IDE can communicate where the compiler is located, but the user can still override this should they choose.

Edit:
With this change compiler redirection will require a shipped version of the targets shim changes which will happen in VS 15.8.

[Pilchie]

But nothing will install to this path anymore, since we removed the MSI?

[KevinRansom]

That is fixed in this PR: #4819

Here:https://github.com/Microsoft/visualfsharp/pull/4819/files#diff-3c8a247250b0ee95f17ac1a0253f962fL1

[brettfo]

I'll have to rebase that PR to fix this bottom line, but this will still work. The MSI removal doesn't remove these files, they're just placed under Common7\IDE\....

[Pilchie]

I guess my overall point is that I'd prefer us to match C# as closely as we can for this stuff (keeping in mind that FSharp.Core throws a small wrench here). I acknowledge that we still need a good way of having the F5'd vsix use the new compiler, but I'd prefer the shipped product mirror C#.

[0x53A]

FSharp.Compiler.Tools overrides the compiler using this property and mirrors Microsoft.Net.Compilers.

Please continue to support both.

[brettfo]

@0x53A Thanks for the pointer. Looking at how FSharp.Compiler.Tools.props works it should be good, but I'll modify my local install of VS to match what this PR does and verify just to be sure.

[brettfo]

@0x53A It works as expected. I did the following:

1. Built these changes locally and deployed the VSIX (to ensure I have the code that sets FSharpCompilerPath on project load).
2. Modified my local targets files to have the redirect that exists in this PR.
3. Built from VS -> fsc was invoked from the deployed VSIX location.
4. I added a reference to FSharp.Compiler.Tools version 10.0.2.
5. Re-built from VS -> fsc was invoked from ..\packages\FSharp.Compiler.Tools\...
6. Re-built from the command line -> same as in step 5.