How can I pass a whitespace-delimited values file as a series of arguments?

Read the file into a an array:

token=($(<list.txt)) yourCommand ${token[@]} 

If the tokens are filenames, containing spaces and separated by newlines, this will not work, because tokens are usually separated by whitespace in the shell.

Assuming that the package names don't contain any wildcard characters (i.e. none of \[?*), you can use a command substitution:

install-packages $(cat list-of-packages.txt) 

Assuming that the package names don't contain any of the characters \"', you can use xargs. Note that this redirects the input of the install-packages command from the package list, so it may not work if the installed requires some interactive input.

xargs install-packages <list-of-packages.txt