Improve depfilemulti warning

Instead of going straight to stderr without any context, instead put the
warning into the output of the action that triggered it, so that it can
be properly attributed when using the protobuf status serializer:

[ 50% 358/708] //external/crosvm/msg_socket/msg_on_socket_derive:libmsg_on_socket_derive rustc
ninja: warning: depfile has multiple output paths (on separate lines); continuing anyway [-w depfilemulti=warn]

In the case where we're loading the depfile at startup (when not using
the deps database), continue using the Warning() to stderr, but at least
prefix the message with which depfile caused the warning.

Test: mmma external/crosvm
Change-Id: I4cf6a9af71596e2d0745408da25ca5e100f50561
7 files changed