lib: rename "self comp. input port message iter." -> "message iterator"

This simplifies the terminology (and therefore the eventual
documentation). It's possible because we have a single type of message
iterator since 6c373cc9 ("lib: remove output port message iterator").

I just moved everything in
`self-component-port-input-message-iterator.h` to `message-iterator.h`
and removed the specific prefix. Header files are about to be reshaped
soon anyway with the C API documentation patch.

In the Python API, I changed *._create_input_port_message_iterator() to
*._create_message_iterator(). I didn't change
`_UserComponentInputPortMessageIterator` because it's not a public name,
so it's not critical to change it now.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I219b25495911363595bdf3b8b3f3b3cf802f20ac
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2749
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>