io/meson.build: move sources to libio_fs.a

Only that one has a dependency on libfs.a (for class AllocatedPath
etc.).

src/io/meson.build

@@ -3,9 +3,7 @@ io = static_library(
   'FileDescriptor.cxx',
   'Open.cxx',
   'PeekReader.cxx',
-  'FileReader.cxx',
   'BufferedReader.cxx',
-  'FileOutputStream.cxx',
   'BufferedOutputStream.cxx',
   include_directories: inc,
   dependencies: [
@@ -16,6 +14,26 @@ io = static_library(
 io_dep = declare_dependency(
   link_with: io,
   dependencies: [
-    fmt_dep,
+  ],
+)
+
+io_fs = static_library(
+  'io_fs',
+  'FileReader.cxx',
+  'FileOutputStream.cxx',
+  include_directories: inc,
+  dependencies: [
+    fmt_dep,
+    io_dep,
+    fs_dep,
+  ],
+)
+
+io_fs_dep = declare_dependency(
+  link_with: io_fs,
+  dependencies: [
+    fmt_dep,
+    io_dep,
+    fs_dep,
   ],
 )