Function: package-compute-transaction

Return a list of packages to be installed, including PACKAGE-LIST.
PACKAGE-LIST should be a list of package names (symbols).

REQUIREMENTS should be a list of additional requirements; each
element in this list should have the form (PACKAGE VERSION-LIST),
where PACKAGE is a package name and VERSION-LIST is the required
version of that package.

This function recursively computes the requirements of the
packages in REQUIREMENTS, and returns a list of all the packages
that must be installed. Packages that are already installed are not included in this list.