feat(pkg): install from branch and subdirectory

[xkevio]

Allows to install a local package from a branch and/or a sub-directory of a git repository.

Adds two new flags to the pkg install sub-command:

• --branch/-b to specify the branch to clone from
• --subdir/-s to specify a relative path to a sub-directory inside the repository

Slightly modifies the clone_git function by adding an optional branch parameter, such that the command resolves to git clone -b <branch> --single-branch <URL> if a branch is specified. The --single-branch ensures that no other branches are fetched.

For --subdir, I used a combination of git clone filter and git sparse-checkout as described in this SO thread.

I have not done extensive testing yet but it has worked in my preliminary testing.