Fix server-33111 spurious split dwarf builds

[bdbaddog]

- Fix spurious rebuilds on second build for cases where builder has > 1 target and the source file
  is generated. This was causing the > 1th target to not have it's implicit list cleared when the source
  file was actually built, leaving an implicit list similar to follows for 2nd and higher target
          ['/usr/bin/python', 'xxx', 'yyy', 'zzz']
  This was getting persisted to SConsign and on rebuild it would be corrected to be similar to this
          ['zzz', 'yyy', 'xxx', '/usr/bin/python']
  Which would trigger a rebuild because the order changed.
  The fix involved added logic to mark all shared targets as peers and then ensure they're implicit
  list is all cleared together.