Symlink include folder instead of files for OpenSSL wrapper

[Flamefire]

(created using eb --new-pr)

Motivation is the PR for Tensorflow where it was observed on at least 2 systems that the system OpenSSL contained an additional include file after an update which makes compilation using the existing includes fail. E.g.:

/mnt/modules/easybuild/software/OpenSSL/3/include/openssl/ssl.h:29:10: fatal error: 'openssl/e_ostime.h' file not found

To avoid having to rebuild and hence temporarily delete the module we should just symlink the include folder instead of the files.

I did some (mostly consistency ) refactoring while digging through the source:

• Use pathlib.Path where paths are stored
• (selected) type hints to make it easier to follow
• self.name in the only place where raw "openssl" was used in the include folder names (we should either use it always or never)
• is_system_ssl property instead of duplicating the same check
• include the warning which variant is used in the log file

Counterargument is this comment by @boegel in 2897480 of #2429:

            # note: symlink to individual files, not directories,
            # since directory symlinks get resolved easily...

It seems to clarify that explicit decision in 9feaf55 see #2429 (comment)
Maybe this does not apply to the headers or not anymore? @lexming

Requires:

• Support pathlib Paths in mkdir and cleanup easybuild-framework#5233
• Fix symlink handling in dir_contains_files easybuild-framework#5236 (otherwise CPATH won't be set)