`fmt` outputs `in` and `every` keyword imports even if `rego.v1` is imported

[johanfylling]

The rego.v1 import implies all future.keyword.* imports, and it's illegal to import these in the same policy.
When a policy has the rego.v1 import and uses any of the in and every keywords, opa fmt will erroneously add imports for future.keywords.in and future.keywords.every; creating a policy that subsequently fails to compile.

E.g. running opa fmt on the following policy:

package test

import rego.v1

a if "foo" in ["foo", "bar"]

b if every x in [1, 2, 3] { x < 4 }

Will generate:

package test

import future.keywords.every
import future.keywords.in

import rego.v1

a if "foo" in ["foo", "bar"]

b if every x in [1, 2, 3] { x < 4 }

which will produce the following error when compiled:

1 error occurred: formatted.rego:6: rego_parse_error: the `rego.v1` import implies `future.keywords`, these are therefore mutually exclusive
	import rego.v1