Bug#952928: [live-build] tidier deb-src inclusion

[jnqnfe@gmail.com]

Package: live-build
Version: 1:20191221
Owner: jnqnfe@gmail.com
Severity: minor

the code for building apt sources.list (which also suffers from a
duplication issue - #952889) is overly messy due to conditionals placed
around every possible inclusion of deb-src entries.

it could be a great deal neater if the conditionals were scrapped, the
deb-src entries thus always written, and then at the end sed was
conditionally used to remove the lines if unwanted.

writing data that only has to then be unwritten "rubs against the
grain", but these are tiny files, only written a couple of times or so
in a build, and the enhanced readability of the code makes this very
much worth it.

patch to be submitted via salsa soon