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/fftw/package.py

@@ -72,6 +72,9 @@ class Fftw(AutotoolsPackage):
     # https://github.com/FFTW/fftw3/commit/902d0982522cdf6f0acd60f01f59203824e8e6f3
     conflicts('%gcc@8:8.9999', when="@3.3.7")
 
+    provides('fftw-api@2', when='@2.1.5')
+    provides('fftw-api@3', when='@3:')
+
     @property
     def libs(self):
 

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