Comment 0 for bug 1811120

[Impact]
Backport auth.conf.d support to allow specifying per-repository authentication data in separate files, so packages can setup authenticated repositories.

[Regression potential]
We ignore errors from opening auth.conf.d files, so regressions can only occur when parsing a file fails, in which case apt would exit with an error.

[Test case]
The test suite provides autopkgtests for auth.conf.d which creates an auth.conf.d file and checks that it is successfully used; so we can check if those passed.