Introduce fftw-api virtual package for Intel-MKL and FFTW (#9618)

* Introduce FFTW2 and FFT3 providers for Intel-MKL and FFTW Spack packages.

* make fftw default package for fftw-api virtual package

* virtual package test assertion now provides location of default virtual packages.

* Change name of virtual package to fftw-api and used versioned interface.

var/spack/repos/builtin/packages/intel-mkl/package.py

@@ -53,6 +53,7 @@ class IntelMkl(IntelPackage):
     provides('lapack')
     provides('scalapack')
     provides('mkl')
+    provides('fftw-api@3', when='@2017:')
 
     if sys.platform == 'darwin':
         # there is no libmkl_gnu_thread on macOS