Previously, `cargo-workspaces` would not detect changes to packages if only
a `[workspace.dependencies]` entry they used was modified in the root
`Cargo.toml`. This was because change detection was solely based on files
modified within the package's own directory.

This commit modifies the `get_changed_pkgs` function to:
1. Check if the root `Cargo.toml` has been modified.
2. If so, parse the old and current versions of the root `Cargo.toml`.
3. Identify which specific dependencies in `[workspace.dependencies]` have been
   added, removed, or had their versions changed.
4. Iterate through all workspace packages, parse their `Cargo.toml` files,
   and determine if they use any of the workspace dependencies that were
   identified as changed.
5. Mark such packages as changed, in addition to those already identified
   by direct file modifications within their directories.

This ensures that packages are correctly identified as changed when their
effective dependencies are altered via the `[workspace.dependencies]` table,
addressing issue #173.