Migrate procfs memory module under procfs/

In practice the `memory.rs` is only used by the procfs sampler. This
commit migrates the code to match its use.

Signed-off-by: Brian L. Troutwine <brian.troutwine@datadoghq.com>

lading/src/observer.rs

@@ -16,9 +16,6 @@ use serde::Deserialize;
 #[cfg(target_os = "linux")]
 mod linux;
 
-#[cfg(target_os = "linux")]
-mod memory;
-
 #[allow(dead_code)] // used on Linux
 /// Expose the process' current RSS consumption, allowing abstractions to be
 /// built on top in the Target implementation.

lading/src/observer/linux/procfs.rs

@@ -1,4 +1,6 @@
 /// Sampler implementation for procfs filesystems
+mod memory;
+
 use std::{collections::VecDeque, io, sync::atomic::Ordering};
 
 use metrics::gauge;
@@ -8,8 +10,8 @@ use procfs::{process::Process, Current};
 use rustc_hash::{FxHashMap, FxHashSet};
 use tracing::{error, warn};
 
-use crate::observer::memory::{Regions, Rollup};
 use crate::observer::RSS_BYTES;
+use memory::{Regions, Rollup};
 
 const BYTES_PER_KIBIBYTE: u64 = 1024;