Use WWW?= to set the default values instead of wrapping the definition into !defined(WWW).

In D36558#831809, @tcberner wrote:

> awk -F '|' '{print $1, $10}' INDEX-14  | grep -e 'http.*://.*http.*://.*'                                                                                                                  
tclgeomap-2.11.6 https://web.archive.org/web/20130807000308/http://www.tkgeomap.org/

[...]

smb_auth-0.05_10 http://web.archive.org/web/20050320094310/http://www.hacom.nl/~richard/software/smb_auth.html
piewm-1.04_4 https://web.archive.org/web/20160409071502if_/http://www.crynwr.com/piewm/index.html~

There still seem to be some issues with the index after the change.

Move definition of default values for ports fetched from GitHub or GitLab into bsd.sites.mk.
Defaults for CPAN, PEAR, and Python are also provided.
Default values are only used if there is no definition in the port or any included USES makefile etc.

In addition to the previous patch provide default WWW values for the USE_GITHUB or USE_GITLAB cases.
Many ports just point at the repository, and in case of GitHub or GitLab we have all required parameters to construct the URL.
Ports can still override this value by defining WWW.

In D36434#829295, @dvl wrote:

Will this be MFC into the latest quarterly branch? I'm going to guess know.

The answer affects FreshPorts changes to cater for make -V WWW - if that won't work on quarterly branches until 2022Q4, I will delay the changes until then.

I had planned to just document the move of the URL from the pkg-descr file to the Makefile.
The new text requests use of HTTPS over HTTP and to add a slash "/" at the end of URLs that point at a host or directory.

The placement of the WWW line has been changed to immediately follow the COMMENT line.