cc/rust: Alias ffi rlib variant for static_libs

Alias the rlib variant to "link: static". This allows declaring
rust_ffi_rlib modules in static_libs. This effectively removes any
distinction between rust_ffi_static and rust_ffi_rlib. Removing the
functionality for building Rust staticlib modules will be cleaned up in
a follow-on CL.

This should have the effect of changing the default linkage for all rust
modules in static_libs from linking individual staticlibs to building a
single staticlib that includes all rust_ffi rlib dependencies.

This removes the static_rlibs property, as we're now handling
the choice dynamically. This also makes rlibs only propagate through
cc_library_static modules if the rlib is included in
whole_static_lib. This both mirrors the expected behavior of
cc libraries and helps control which version of a crate ends up in the
final link (e.g. libdoh_ffi vs libdoh_ffi_for_test).

Bug: 254469782
Test: m
Test: m blueprint_tests
Change-Id: I2925f67f6dc9329dae3dcccafb8560900ac8a6fc

cc/linkable.go

@@ -94,12 +94,16 @@ type LinkableInterface interface {
 	SelectedStl() string
 
 	BuildStaticVariant() bool
+	BuildRlibVariant() bool
 	BuildSharedVariant() bool
 	SetStatic()
 	SetShared()
 	IsPrebuilt() bool
 	Toc() android.OptionalPath
 
+	// IsRustFFI returns true if this is a Rust FFI library.
+	IsRustFFI() bool
+
 	// IsFuzzModule returns true if this a *_fuzz module.
 	IsFuzzModule() bool