Porting/Maintainers.pl - Remove most MAINTAINER fields

Remove the MAINTAINER field if it was 'p5p' (or 'blead', which was the case for
Unicode::UCD, presumably in error) or if there is a DISTRIBUTION field. Only a
few MAINTAINER fields are now left, for modules with non-'p5p' maintainers but
no CPAN distribution. For modules now having no MAINTAINER field, the maintainer
is understood to be either the Perl 5 Porters if there is no DISTRIBUTION field
or the UPSTREAM field is 'blead', or else the CPAN author whose PAUSE user ID
forms the first part of the DISTRIBUTION value.

Set up a legacy MAINTAINER field for backwards compatibility with existing
functionality in Porting/Maintainers, Porting/corecpan.pl and
Porting/corelist.pl.

The latter program now omits modules such as Attribute::Handlers from %upstream
and %bug_tracker in CoreList.pm because the MAINTAINER is now understood to be
'p5p' since the UPSTREAM field is 'blead'. Previously it was included because a
non-'p5p' MAINTAINER was given, but that was really at odds with UPSTREAM being
'blead'.
Porting/Maintainers.pl - Use PAUSE user IDs

Since we now encourage the use of the DISTRIBUTION field to find the PAUSE
user ID of the maintainer rather than explicitly listing most maintainers,
it makes sense to also use PAUSE user IDs where possible for the few
MAINTAINERS that are still given explicitly.
Porting/Maintainers.pl - Consistent quotes
Porting/Maintainers.pl - No need to list IGNORABLES as EXCLUDED
Porting/corelist.pl - Fix breakage since commit c676c8385f