splitmuxsrc: Refactor part preparation code and remove "prepared" signal from reader helper object

We don't need a special signal anymore but can directly work with
async-done
8 jobs for splitmuxsrc-async in 135 minutes and 17 seconds (queued for 3 minutes and 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #76602
gst indent

00:00:31

passed #76601
manifest

00:00:39

 
  Build
passed #76603
build fedora x86_64

00:05:31

 
  Test
passed #76604
check fedora

00:18:21

 
  Full Builds
passed #76605
build android arm64 api28

00:04:14

passed #76607
build cerbero cross win32

00:49:45

passed #76608
build cerbero cross win64

00:46:15

passed #76606
build cerbero fedora x86_64

00:52:44