Issue 4870: [midi2ly] Delay import of 'midi' module.

Issue 4870: [midi2ly] Delay import of 'midi' module.

`make all` starts with generating the manual pages for lilypond's python
scripts using help2man.  However, at the time `midi2ly --help` gets called,
the 'midi' python module (which is based on code written in C) imported by
'midi2ly' has not been compiled yet, making help2man abort.