Allow Python module name to import to be overridden

Some Python module names undergo a lossy transformation to be mapped
to Debian package names. For example, the Xlib module has to be packaged
as python3-xlib, which is not enough information for autodep8 to infer
that it should `import Xlib`.

Some Debian Python packages have their name based on their PKG-INFO
or .egg-info files rather than the name that is to be imported, which
means autodep8 cannot guess the right name to import. For example,
python3-pypcap is to be used via `import pcap`. This is at least
arguably incorrect, but is somewhat common.

Similarly, some packages are named with a non-canonical transformation
of their Python module name, for example the oslo family of packages
(#884181) which are packaged with names like python3-oslo.log but
imported with a name like oslo_log.

Taking inspiration from the Perl autopkgtest in pkg-perl-tools, I've
allowed the import-name file to include comments and blank lines
(which are ignored), or even multiple lines (of which only the first
is used for now).

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #884181
Closes: #929957