package-installs for pre-install.d/post-install.d

Packages are often also installed in both the pre-install.d and
post-install.d phases. This patch expands the package-installs element
to support declarative package support for these phases in addition to
the existing support for install.d. The actual install/uninstall logic
is moved to common scripts under bin/ so that it can be reused across
the different phases.

Change-Id: Id51d0bbad232737fc8b5ffaf016dec50cd5b66c9
Merge "package-installs for pre-install.d/post-install.d"