Update packages which need include subdirectories

[scheibelp]

Fixes #10769
Closes #10751 (EDIT: see #10773 (comment))

@HadrienG2 @davydden @jslee02 @alalazo

#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.

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

In hindsight, it was a mistake not to request documentation on #10623. So that should be added here or soon.

[scheibelp]

@jslee02 regarding #10751 I wanted to confirm myself that this fixes but didn't actually see packages using .headers for the eigen package.

If this works for you, it would be the ideal way of including subdirectories (over modifying CPATH). Are you using a custom package which calls Eigen.headers (no problem with that, I just wanted to make sure I wasn't overlooking an example of that access in an existing Spack package).

[HadrienG2]

Thanks, I finally understood why #10623 broke python in spite of it already overriding headers. Turns out one needs to override both headers AND to manually reset the HeaderList's directories property.

I think extra documentation would definitely be helpful here. Not sure where it would be most visible to those who need it, though.

In any case, if you haven't tested this with boost+python already, I will give it a try myself next week 😉

[HadrienG2]

Works for me 👍

[alalazo]

@scheibelp If you prefer having a separate PR for docs I'd merge this.

[jslee02]

@scheibelp I use scons as a buildsystem for my project and run spack module loads <packages> to let scons find the package location using the environment variables (e.g., CPATH). Could you please elaborate how this PR fixes #10751 without modifying CPATH?