Implement post-install hooks

[thetredev]

For example, when installing vscode, the code shell command is not present on Void Linux. Linking /usr/share/code/bin/code to /usr/bin/code or /usr/local/bin/code has to be done manually. This should be automated. The tool needs to be aware of this and handle it appropriately, but the actual definition of what needs to be done has to be specified in the package definition over at https://github.com/thetredev/xdeb-install-repositories.

[thetredev]

@toluschr could this be the solution to post_unpack? Or is post_unpack supposed to be executed before package installation?

[thetredev]

Implemented with 9b7d7d3