Rule: suggest importing commonly used `data` ref

[johanfylling]

If a module has more than one reference to the same data document, and there is no corresponding import, the largest common path should be suggested as a new import.

E.g., in the policy:

package example

p := data.a.b.c

q {
  data.a.b.d == 2
}

two "naked" references to the data.a.b document are made. The preferred style should be to import this reference:

package example

import data.a.b

p := b.c

q {
  b.d == 2
}

Note: direct imports of a rule or function is likely not preferred, as some sort of grouping/namespacing might be implied, so even if several references are made to the same rule/function, it's parent document should be imported, e.g. for the policy:

package example

p := data.a.b.c

q {
  data.a.b.c == 2
}

an import for the data.a.b document should be suggested, not data.a.b.c.