Mapping or migrating path alias using using Migrate Source CSV
@Jigar's answer is close but not quite. Here is what I eventually figured out. It should be path/alias and not just path. Also, if you have pathauto module installed it will create the path based on the patterns. So, in order for our path to be imported from csv, the path auto needs to be disabled first. You set path/pathauto's default value to 0 to do that. 

source:
...
 constants:
  slash: '/'
....
process:
...
 path/pathauto: 
   plugin: default_value
   default_value: 0 # Disable pathauto
 path/alias: 
   plugin: concat
   source:
     - constants/slash
     - url