Distinguish the intent of PackagingSpec

A PackagingSpec actually means either of two: an installation of a file
(i.e. putting the file on a filesystem image), or a simple gathering.
However, so far, the two different intents was not visible in
PackagingSpecs returns from TransitivePackagingSpecs().

With this change, the two different intents are recorded in
PackagingSpec. A PackagingSpec has a boolean field which records the
value of the SkipInstall() of the module where the PackagingSpec was
created from.

This could be used by a module like android_system_image which wants to
follow the "installation" semantic. For example, the module type doesn't
want to include a stub variant of a shared lib.

This change doesn't make use of the new field, yet.

Bug: 335506668
Test: go test ./...
Change-Id: If0106642a11ba804630df79b4c5ebd608a77a6b1