better diagnostic when filter excludes all

[plicease]

Caveat: the filter stage should not modify the $res passed in, but that doesn't actually stop it from doing that :/

before:

Alien::Build::Plugin::Core::Download> decoding html
no matching files in listing at /home/ollisg/opt/perl/5.37.5/lib/site_perl/5.37.5/Alien/Build/Plugin/Core/Download.pm line 61.
Recipe did not seem to download a file or directory

after:

Alien::Build::Plugin::Core::Download> decoding html
Alien::Build::Plugin::Core::Download> These files were excluded by the filter stage:
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/?C=N;O=D
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/?C=M;O=A
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/?C=S;O=A
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/?C=D;O=A
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/README.older-versions
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.75-3.76.diff.gz
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.75.tar.gz
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.76-3.76.1.diff.gz
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.76.1-3.77.diff.gz
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.76.1.tar.gz
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.77.tar.gz
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.78.1.tar.gz
Alien::Build::Plugin::Core::Download> excluded https://ftp.gnu.org/gnu/make/make-3.79.1.tar.gz
Alien::Build::Plugin::Core::Download> excluded ...
no matching files in listing at /home/ollisg/opt/perl/5.37.5/lib/site_perl/5.37.5/Alien/Build/Plugin/Core/Download.pm line 61.
Recipe did not seem to download a file or directory