Only use URI from mirror where we found the module and not from all m…

[perlancar]

…irrors.

When multiple --mirrors are used, cpan_dist() will construct URIs from all
mirrors, regardless or whether this mirror contains the module or not. This can
cause a lot of noise warnings of failing to fetch files from the mirrors,
particularly when darkpan(s) are involved. For example:

% cpanm --mirror http://darkpan1/ --mirror http://darkpan2/ \
    --mirror http://darkpan3/ --mirror http://cpan/ --mirror-only \
    A-CPAN-Module-1.23.tar.gz
! Finding A::CPAN::Module () on mirror http://darkpan1/ failed.
! Finding A::CPAN::Module () on mirror http://darkpan2/ failed.
! Finding A::CPAN::Module () on mirror http://darkpan3/ failed.
--> Working on A::CPAN::Module
Fetching http://darkpan1/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz ... FAIL
! Download http://darkpan1/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
! Download http://darkpan1/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
! Download http://darkpan1/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
Fetching http://darkpan2/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz ... FAIL
! Download http://darkpan2/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
! Download http://darkpan2/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
! Download http://darkpan2/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
Fetching http://darkpan3/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz ... FAIL
! Download http://darkpan3/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
! Download http://darkpan3/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
! Download http://darkpan3/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz failed. Retrying ...
Fetching http://cpan/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz ... OK
...

With this small commit, the distribution A-CPAN-Module will only get the
http://cpan/ URL as that is where the associated module is found. So, the noise
is much reduced:

% cpanm --mirror http://darkpan1/ --mirror http://darkpan2/ \
    --mirror http://darkpan3/ --mirror http://cpan/ --mirror-only \
    A-CPAN-Module-1.23.tar.gz
! Finding A::CPAN::Module () on mirror http://darkpan1/ failed.
! Finding A::CPAN::Module () on mirror http://darkpan2/ failed.
! Finding A::CPAN::Module () on mirror http://darkpan3/ failed.
--> Working on A::CPAN::Module
Fetching http://cpan/authors/id/P/PA/PAUSEID/A-CPAN-Module-1.23.tar.gz ... OK
...

[miyagawa]

This looks reasonable to me - is it possible to write a unit test in xt ? there are several existing tests that uses a local directory as a mirror source.