InstallPath keeps its partition dir

This change introduces the concept of partition dir for InstallPaths.
It's the path to the partition where the InstallPath is rooted at. For
example, it's out/soong/target/product/<device>/<partitoon> for paths
created for device modules. For host modules, it is defined as
out/soong/host/<host_os>-<host_arch>.

The partition dir is obtained using the new PartitionDir() function.

Another change is that a freshly created InstallPath (usually via
PathForModuleInstall) is the result of joining PartitionDir() and the
remaining path elements. For example, PathForModuleInstall(ctx, "foo",
"bar").Rel() now returns "foo/bar". Previously, that call returned the
relative path from config.buildDir() ("out/soong"). This change is in
line with the behavior of other path-creating functions like
PathForModuleSrc where Rel() returns the path relative to the
contextually determined path like the module source directory.

Notice that the Join() call to InstallPath doesn't change
PartitionDir(), while does change the result of Rel().

p := PathForModuleInstall(ctx, "foo", "bar")
p.PartitionDir() is out/soong/host/linux-x86
p.Rel() is foo/bar

q := p.Join(ctx, "baz")
q.PartitionDir() is still out/soong/host/linux-x86
q.Rel() now returns baz

Bug: N/A
Test: m nothing
Change-Id: I916bb1c782a4bfe0fbd4854e349cd2a2a42f56b6