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