Improve Deb822 parser coverage

[SanchezTheCoder]

This adds a focused follow-up for the Deb822 branch:

• wires test_deb822_integration into make check
• preserves comments and optional Deb822 fields while parsing
• treats whitespace-only lines as stanza separators
• removes leftover repository dialog debug prints
• fixes an autotools trailing-backslash warning in common/Makefile.am

Verified in a Debian testing container:

./autogen.sh
./configure --prefix=/usr
make -j2
make check -j2

make check now runs and passes test_deb822_integration.

[Peter1169]

I found one more small edge case while testing this branch: ReadSources() can load the same .sources file twice when Dir::Etc::sourcelist.d and Dir::Etc::sourceparts resolve to the same directory.

Opened an incremental PR against this branch here: SanchezTheCoder#1

It removes the duplicate Deb822 scan and adds a regression test for the duplicate-record case. I verified it in a Debian testing container with ./autogen.sh, ./configure --prefix=/usr, make -C common -j2, make -C tests -j2 test_deb822_integration, and ./tests/test_deb822_integration (11 tests passing).