Fixes https://github.com/mono/mono/issues/10602 .
From the issue:
```
We need to enable this to support the system assemblies conflict
resolution which we now rely on for any new packages to enhance
developers experience and get us out of dependency on specific package
versions.
```
.. MSBuildExtensionsPath* .
`MSBuildExtensionsPath*` supports search paths only if the string
`$(MSBuildExtensionsPath)` (or *32/*64) appear in the `Project`
attribute of the `Import` element. Otherwise the value of the property
or the default value is used. So, restructure to conditionally import
rather than a import using a property with the final path.
Mono's fork of msbuild uses a `$(_DebugFileExt)` property to specify
the debug file extension (.pdb/.mdb) to use. It defaults to .pdb. So,
with XI/XM, .mdb files don't get copied to the output folder.
We also add a `$(FscDebugFileExt)` property, which allows our default of
`.mdb` to be overridden.
But the `$(_DebugFileExt)` support is not in upstream msbuild yet. So,
we can't ask the F# upstream to add it. Hence, this is being added to
our FSharp target files. Once, all this is upstream, we can remove the
overrides from our files.
Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=51148
Rolf Bjarne Kvinge
Timothé Larivière
Ankit Jain
Don Syme
Ankit Jain
nosami