Support declarative package installs/uninstalls

Adds a new element, package-installs, that provides an interface for
declarative package installs and uninstalls. Packages to install can be
added to an install.d/package-installs-<element-name> file. The set of
packages listed across such files are installed in a single transaction
at the beginning of install.d.

Prefacing the package name with a "-" indicates that the package should
be uninstalled at the end of the install.d phase. Again, the full set of
uninstalls are done in a single transaction.

An element providing a package-installs file should add package-installs
to its element-deps file.

Change-Id: I5b540388eff1079c8dee933b869463371481152b
Merge "Support declarative package installs/uninstalls"