Replace "repeatable" by "variable" (in VirtualFiles and maybe tasks)

[YPares]

VirtualFiles are called "repeatable" when their paths are to contain variables to be spliced in. It occured to me that this is actually more generic than passing around file indices.

For instance this could be used to determine dynamically the entire path of a file from the content of another file:

locations:
  /inputs/stuff: "{stuffPath}"

So "repeatable" is too specific. "Variable" would encompass better the uses.