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
Fixes: 144948629
Change-Id: I8409e43011a4754e095ad1b368570a2ba8d75e50
Merged-In: I8409e43011a4754e095ad1b368570a2ba8d75e50
(cherry picked from commit 1a527688)