Add required, host_required, and target_required as dependencies

So far, the installation of required modules were handled by Make. This
prevents us from implementing the module installation and packaging
entirely in Soong.

This CL is the first step towards that goal. Soong now correctly tracks
the dependencies and they are correctly returned by
TransitivePackagingSpecs(), which is used by packaging modules like
android_system_image.

Bug: 321626681
Test: build
Change-Id: I9192b5333ceaa0b7d1c5c4abeec2af62febcd976

cc/cc.go

@@ -2980,6 +2980,9 @@ func checkDoubleLoadableLibraries(ctx android.TopDownMutatorContext) {
 		if depTag == stubImplDepTag {
 			return false
 		}
+		if depTag == android.RequiredDepTag {
+			return false
+		}
 
 		// Even if target lib has no vendor variant, keep checking dependency
 		// graph in case it depends on vendor_available or product_available
@@ -3157,6 +3160,10 @@ func (c *Module) depsToPaths(ctx android.ModuleContext) PathDeps {
 			return
 		}
 
+		if depTag == android.RequiredDepTag {
+			return
+		}
+
 		if dep.Target().Os != ctx.Os() {
 			ctx.ModuleErrorf("OS mismatch between %q and %q", ctx.ModuleName(), depName)
 			return