Header subdirectories for Python and Eigen (#10773)

Fixes #10769 

This updates the .headers property to include header subdirectories
for Python and Eigen (as is recommended by these packages).

#10623 updated the default behavior of .headers.directories to
exclude subdirectories (since this can cause clashes with system
headers). This broke some packages which depended on the old behavior
of .headers.directories: for example if you had
<package-prefix>/include/subdir/ex1.h, .headers.directories would
include <package-prefix>/include/subdir.

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

@@ -46,3 +46,9 @@ class Eigen(CMakePackage):
     depends_on('gmp', when='+mpfr')
 
     patch('find-ptscotch.patch', when='@3.3.4')
+
+    @property
+    def headers(self):
+        headers = find_all_headers(self.prefix.include)
+        headers.directories = [self.prefix.include.eigen3]
+        return headers

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

@@ -619,13 +619,15 @@ def libs(self):
     def headers(self):
         config_h = self.get_config_h_filename()
 
-        if os.path.exists(config_h):
+        if not os.path.exists(config_h):
-            return HeaderList(config_h)
-        else:
             includepy = self.get_config_var('INCLUDEPY')
             msg = 'Unable to locate {0} headers in {1}'
             raise RuntimeError(msg.format(self.name, includepy))
 
+        headers = HeaderList(config_h)
+        headers.directories = [os.path.dirname(config_h)]
+        return headers
+
     @property
     def python_lib_dir(self):
         return join_path('lib', 'python{0}'.format(self.version.up_to(2)))