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/pexsi/package.py

@@ -67,8 +67,8 @@ def install(self, spec, prefix):
             '@PARMETIS_PREFIX': self.spec['parmetis'].prefix,
             '@LAPACK_PREFIX': self.spec['lapack'].prefix,
             '@BLAS_PREFIX': self.spec['blas'].prefix,
-            '@LAPACK_LIBS': self.spec['lapack'].lapack_shared_lib,
-            '@BLAS_LIBS': self.spec['lapack'].blas_shared_lib,
+            '@LAPACK_LIBS': self.spec['lapack'].lapack_libs.joined(),
+            '@BLAS_LIBS': self.spec['lapack'].blas_libs.joined(),
             '@STDCXX_LIB': ' '.join(self.compiler.stdcxx_libs)
         }