Support for finding changelog.Debian.gz in perl-base

[Simon Ruggier]

Hi all, thanks for making it easy to manipulate Debian packages in Python
code. Debian's perl packages in testing have an unusual layout: the perl
package has only an upstream changelog, while the changelog.Debian.gz is at
/usr/share/doc/perl/changelog.Debian.gz within the perl-base package. Right
now, DebFile fails to find the changelog, because it's looking for it at
/usr/share/doc/perl-base/changelog.Debian.gz.

To support this configuration without adding a special case for perl-base,
I propose checking /usr/share/doc/<source package name>/changelog.Debian.gz
before checking the native changelog path.

In the future, if any packages are more unusual than this, I think the next
step would be to iterate over all of the files in the package looking for
/usr/share/doc/*/changelog.Debian.gz, but I didn't want to potentially harm
performance, so I didn't go that far.

I've attached two patches with cleanups near the code involved, as well as
a third patch that implements this suggestion. I've tested the updated code
on my system and successfully parsed changelog files out of both perl-base
and other packages that implement normal changelog parsing.

Cheers,
Simon
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