build_packages.sh: Migrate revdeps to Python

Get and parse base install packages and reverse dependencies packages
passed as envvars from the Python script.

BUG=b:218521900
TEST=CQ

Cq-Depend: chromium:3588861
Change-Id: I07deb176101be752ec28e7a93d1f258c228b921d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3654199
Tested-by: Cindy Lin <xcl@google.com>
Commit-Queue: Cindy Lin <xcl@google.com>
Reviewed-by: Ram Chandrasekar <rchandrasekar@google.com>
1 file changed