app: separate interface from implementation

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/app_stub_test.go

@@ -7,7 +7,7 @@ import (
 	"os/user"
 	"strconv"
 
-	"git.gensokyo.uk/security/fortify/internal/sys"
+	"git.gensokyo.uk/security/fortify/fst"
 )
 
 // fs methods are not implemented using a real FS
@@ -126,8 +126,8 @@ func (s *stubNixOS) Open(name string) (fs.File, error) {
 	}
 }
 
-func (s *stubNixOS) Paths() sys.Paths {
-	return sys.Paths{
+func (s *stubNixOS) Paths() fst.Paths {
+	return fst.Paths{
 		SharePath:   "/tmp/fortify.1971",
 		RuntimePath: "/run/user/1971",
 		RunDirPath:  "/run/user/1971/fortify",