Reworking of lapack_shared_libs and similar properties (#1682)

* Turned <provider>_libs into an iterable

Modifications :
- added class LibraryList + unit tests
- added convenience functions `find_libraries` and `dedupe`
- modifed non Intel blas/lapack providers
- modified packages using blas_shared_libs and similar functions

* atlas : added pthread variant

* intel packages : added lapack_libs and blas_libs

* find_library_path : removed unused function

* PR review : fixed last issues

* LibraryList : added test on __add__ return type

* LibraryList : added __radd__ fixed unit tests

fix : failing unit tests due to missing `self`

* cp2k and dependecies : fixed blas-lapack related statements in package.py

var/spack/repos/builtin/packages/elk/package.py

@@ -87,12 +87,12 @@ def configure(self, spec):
         # BLAS/LAPACK support
         # Note: BLAS/LAPACK must be compiled with OpenMP support
         # if the +openmp variant is chosen
-        blas   = 'blas.a'
+        blas = 'blas.a'
         lapack = 'lapack.a'
         if '+blas' in spec:
-            blas   = spec['blas'].blas_shared_lib
+            blas = spec['blas'].blas_libs.joined()
         if '+lapack' in spec:
-            lapack = spec['lapack'].lapack_shared_lib
+            lapack = spec['lapack'].lapack_libs.joined()
         # lapack must come before blas
         config['LIB_LPK'] = ' '.join([lapack, blas])