List of packages for `package-initialize' to load.
Each element in this list should be a list (NAME VERSION), or the
symbol `all'. The symbol `all' says to load the latest installed
versions of all packages not specified by other elements.
For an element (NAME VERSION), NAME is a package name (a symbol).
VERSION should be t, a string, or nil.
If VERSION is t, all versions are loaded, though obsolete ones
will be put in `package-obsolete-alist' and not activated.
If VERSION is a string, only that version is ever loaded.
Any other version, even if newer, is silently ignored.
Hence, the package is "held" at that version.
If VERSION is nil, the package is not loaded (it is "disabled").