Create Your Own Python Package Manager (Simplified)

This challenge asks you to build a foundational piece of a Python package manager, focusing on the core logic of finding, downloading, and installing packages. Understanding this process is crucial for anyone working with Python, as it underpins how you manage dependencies for your projects.

Problem Description

Your task is to create a Python script that simulates a simplified version of pip. This script will take a package name as input, find it on a simulated package index, download it, and "install" it by placing it in a designated "site-packages" directory.

Key Requirements:

1. Package Index: You will need to simulate a package index. For this challenge, a simple dictionary mapping package names to their versions and download URLs will suffice.
2. Package Resolution: Given a package name, the script should look it up in the simulated index.
3. Download: The script should simulate downloading a package from its given URL. In practice, this would involve fetching a file (like a .whl or .tar.gz); for this challenge, you can represent this by simply noting that the download occurred.
4. Installation: The script should simulate installing the package by moving it into a predefined "site-packages" directory. Again, for simplicity, you can represent this by creating a placeholder file named after the package in this directory.
5. Dependency Handling (Optional, for advanced users): A more advanced version could handle simple dependencies, where packages list other packages they require. If a dependency is not found, it should attempt to install it first.
6. Error Handling: The script should gracefully handle cases where a package is not found in the index.

Expected Behavior:

When run with a valid package name, the script should report that it has found, downloaded, and installed the package. If the package is not in the index, it should report that it could not be found.

Edge Cases:

• Package not found in the index.
• Simulated index is empty.

Examples

Example 1:

Input:
package_name = "requests"
simulated_index = {
    "requests": {
        "version": "2.28.1",
        "url": "http://example.com/packages/requests-2.28.1.tar.gz"
    },
    "urllib3": {
        "version": "1.26.12",
        "url": "http://example.com/packages/urllib3-1.26.12.tar.gz"
    }
}
site_packages_dir = "./my_site_packages"

# Expected Output (simulated):
# Found package 'requests' in index.
# Simulating download of http://example.com/packages/requests-2.28.1.tar.gz for requests.
# Installed 'requests' into ./my_site_packages/requests.

Explanation:

The script successfully finds "requests" in the simulated_index, prints messages indicating download and installation, and creates a placeholder file.

Example 2:

Input:
package_name = "nonexistent_package"
simulated_index = {
    "requests": {
        "version": "2.28.1",
        "url": "http://example.com/packages/requests-2.28.1.tar.gz"
    }
}
site_packages_dir = "./my_site_packages"

# Expected Output (simulated):
# Package 'nonexistent_package' not found in the index.

Explanation:

The script correctly identifies that "nonexistent_package" is not present in the simulated_index and reports this to the user.

Constraints

• The simulated_index will be a Python dictionary.
• Package names will be strings.
• The site_packages_dir will be a string representing a directory path.
• Your script should execute within reasonable time limits (no complex algorithms are required here).
• For simplicity, do not implement actual file downloading or complex file system operations beyond creating a directory and a placeholder file.

Notes

• Think about how you would structure your code. A class representing the package manager might be a good approach.
• For the "download" step, printing a descriptive message is sufficient.
• For the "install" step, creating a dummy file in the site_packages_dir is enough to signify successful installation. For example, if installing "requests", create a file named requests within the site_packages_dir.
• Consider using os.makedirs to create the site_packages_dir if it doesn't exist.
• For the advanced dependency handling, you can assume a simple dependency structure within your simulated_index (e.g., each package entry could have an optional "dependencies" key which is a list of package names).