Support for split layers

Closes: ostreedev#69

This is initial basic support for splitting files (objects) from
a commit into separate container image layers, and reassembling
those layers into a commit on the client.

We retain our present logic around e.g. GPG signature verification.

There's a new `chunking.rs` file which has logic to automatically
factor out things like the kernel/initramfs and large files.

In order to fetch these images client side, we now heavily
intermix/cross the previous code for fetching non-ostree layers.

lib/Cargo.toml

@@ -10,7 +10,7 @@ version = "0.6.5"
 
 [dependencies]
 anyhow = "1.0"
-containers-image-proxy = "0.4.0"
+containers-image-proxy = { features = ["proxy_v0_2_3"], version = "0.4.0" }
 
 async-compression = { version = "0.3", features = ["gzip", "tokio"] }
 bitflags = "1"
@@ -58,4 +58,3 @@ features = ["dox"]
 [features]
 dox = ["ostree/dox"]
 internal-testing-api = ["sh-inline", "indoc"]
-proxy_v0_2_3 = ["containers-image-proxy/proxy_v0_2_3"]

lib/src/bootabletree.rs

@@ -0,0 +1,29 @@
+//! Helper functions for bootable OSTrees.
+
+use anyhow::Result;
+use ostree::gio;
+use ostree::prelude::*;
+
+const MODULES: &str = "/usr/lib/modules";
+
+/// Find the kernel modules directory in a bootable OSTree commit.
+pub fn find_kernel_dir(
+    root: &gio::File,
+    cancellable: Option<&gio::Cancellable>,
+) -> Result<Option<gio::File>> {
+    let moddir = root.resolve_relative_path(MODULES);
+    let e = moddir.enumerate_children(
+        "standard::name",
+        gio::FileQueryInfoFlags::NOFOLLOW_SYMLINKS,
+        cancellable,
+    )?;
+    let mut r = None;
+    for child in e.clone() {
+        let child = &child?;
+        let childpath = e.child(child);
+        if child.file_type() == gio::FileType::Directory && r.replace(childpath).is_some() {
+            anyhow::bail!("Found multiple subdirectories in {}", MODULES);
+        }
+    }
+    Ok(r)
+}