rust: rust_proc_macro host snapshot support.

Adds support for capturing rust_proc_macros as part of the host
snapshot. Proc macros target the host and can be thought of as compiler
plugins. Because of this, they don't have vendor image variants and
can't be easily captured as part of the vendor snapshot. Instead we
capture them as part of the host snapshot.

This adds a rust_prebuilt_proc_macro module type.

Bug: 204304380
Test: m HOST_FAKE_SNAPSHOT_ENABLE=true host-fake-snapshot dist
Test: python3 development/vendor_snapshot/update.py --image=host
  --install-dir=vendor/vendor_name/ 31 --local out/dist
Test: Checked Android.bp for rust_prebuilt_proc_macro modules.
Change-Id: I4a8c4d9c41b7ca361b5b97d3f74973918c2a5fe3

snapshot/snapshot_base.go

@@ -114,6 +114,8 @@ type SnapshotJsonFlags struct {
 	RelativeInstallPath string `json:",omitempty"`
 	Filename            string `json:",omitempty"`
 	ModuleStemName      string `json:",omitempty"`
+	RustProcMacro       bool   `json:",omitempty"`
+	CrateName           string `json:",omitempty"`
 
 	// dependencies
 	Required []string `json:",omitempty"`