refactor: split ecr into library and cli crates

The root package becomes a virtual workspace: `crates/ecr` holds the
library (package name `ecr`) and `crates/ecr-cli` the command line
front-end, which keeps installing the `ecr` binary. No behavior change.

The library must not depend on CLI types, so mount::setup_mounts now
takes the `no_bind` flag instead of a `&Args`.

crates/ecr-cli/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "ecr-cli"
+description = "Enter chroot environments with Linux namespaces"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+authors.workspace = true
+
+# The installed binary keeps the `ecr` name even though the CLI package is
+# `ecr-cli`; the library package owns the `ecr` name for consumers.
+[[bin]]
+name = "ecr"
+path = "src/main.rs"
+
+[dependencies]
+ecr = { path = "../ecr" }
+
+# CLI parsing
+clap = { version = "4", features = ["derive", "env"] }
+
+# Error handling
+anyhow = "1"
+
+# Interim: used by the CLI's inline cache/extraction orchestration until it
+# moves behind the library's rootfs API
+dirs = "6"
+tempfile = "3"