/etc/apache2/mods-available/auth_plain.load is in libapache2-mod-auth-plain 2.0.52.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # Depends: authn_core authz_core
LoadModule auth_plain_module /usr/lib/apache2/modules/mod_auth_plain.so
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