Add PackageFile and use it for prebuilt build tools

Add PackageFile that creates a PackageSpec like InstallFile,
but doesn't create the install rule.  Also improve the documentation
on InstallFile and friends, and on HostToolProvider.

Bug: 124313442
Test: m checkbuild
Change-Id: I30f44a2b3833e48d0bb71bf7c45955f88fb00bc8