...it seems that within the "download and install environment" the .dist package can find the two "subsidiary" packages to which it points...
The .dist file is not a package; it's an XML file (36 KB in the case of the
iTunesX-7.6.2.dist file that I referred to in my own case above). Just for grins, if the update downloads in the same form on your backup, take a peek at it using Quick Look. If that works the way it did in my case, you'll see that the document consists of the Apple Software License Agreement followed by a collection of JavaScript functions such as
InstallationCheck,
ShouldAutoUpdate, and
currentReceiptExists.
The .dist file, IOW, is simply an installation script, which tells Installer how to choreograph the user interface events of the installation of the (meta)package within which the .dist file resides. However, in the cases under discussion, there is no "outer" package, only a folder. It's my hypothesis that that folder (
iLife Support in your case) is really an "unbundled" distribution package, as described earlier. If that's the case, your download wasn't missing any of its constituent
parts (and thus you were able to install the two packages); it was only missing the
"glue" by which the "outer" package should have been displayed as a single, clickable .mpkg icon.
The real problem with this type of scenario is that since the .dist script is now no longer overriding the individual installation scripts contained within the individual packages, in those cases in which the installation sequence matters it's possible for the user manually installing the packages to get the sequence wrong.
As for how it is that the distribution metapackage becomes "unbundled," I'm not sure. The Mac Dev Center's
About Bundles ->
Bundles and Packages ->
How the System Identifies Bundles and Packages is worth a look.
(I experimented by removing the .mpkg extension from
GoogleEarthPlugin.mpkg, the one archived .mpkg I had available, and the metapackage became a folder, as expected, but that's not conclusive; it only demonstrates that removal of the extension
can be a cause of "unbundling," not that it's the cause in the cases under discussion. Perhaps Hal will contribute an explanation of the "package bit," which I lack the time to investigate at the moment. In any event—according to my hypothesis—
something in the
Download Only process changes the package to an ordinary folder. It's not inconceivable that this flaw occurs frequently enough that it provoked Apple into removing the
Download Only option in Snow Leopard, but we'll likely never know. The option reportedly still exists within
softwareupdate from the command line.)