Shard gensrcs modules into multiple commands

gensrcs modules use a single command to convert all of the sources,
which can lead to command line length limits, long critical path
times, and excessive rebuilds.  Shard them into multiple rules,
defaulting to 100 commands in each.

Test: TestGenSrcs
Test: m
Fixes: 70221552
Fixes: 138438756
Fixes: 138829091
Change-Id: I8409e43011a4754e095ad1b368570a2ba8d75e50