Removes mergechanges step and changes usage of changes files

 * added --source-only-changes to pbuilder options
 * altered valid .changes count to exactly one, the _source.changes file
 * removed mergechanges step
 * changed lintian profile to pureos/main
1 job for issues/37 in 25 seconds (queued for 1 minute and 8 seconds)
latest
Status Job ID Name Coverage
  Test
passed #246302
librem5
build-native

00:00:25