Refactor and enhance source management and installation processes

- Improved error handling and context messages in `checkout` function in `git.rs`.
- Updated license field in package specification to use a vector in `hooks.rs`.
- Enhanced `copy_manual_sources` function in `mod.rs` to support both local file and remote URL sources, including checksum verification.
- Added new utility functions for path normalization and skipping installation of specific files in `staging/mod.rs`.
- Implemented logic to handle existing files during installation, allowing for `.depotnew` suffix for kept files.
- Added tests for manual source copying, installation behavior, and path validation.
- Introduced a new LICENSE file with MIT License terms.

src/source/hooks.rs

@@ -253,7 +253,7 @@ mod tests {
                 revision: 1,
                 description: "d".into(),
                 homepage: "h".into(),
-                license: "MIT".into(),
+                license: vec!["MIT".into()],
             },
             packages: Vec::new(),
             alternatives: Alternatives::default(),