Update the Rego validation when functions are imported in a policy

[santoshkal]

Update the Rego validation logic to parse the .rego policy file which has imported another .rego function from different packages.

Example:

# ./rego/k8s/check_cpu_limits.rego
#  policy to check if  resource limits are defined  in Deployment

package validate_k8s

import rego.v1
import data.k8s.common as lib # importing a .rego function from different package


check_cpu_limits[msg]{
input.kind ==	"Deployment"
c:= input.spec.template.spec.containers[i]
not lib.valid_key(c.resources.limits,"cpu") # using the `valid_key` func from `lib` package 
msg:="CPU Limits are set for Deployment"
}  

Now package is defined in different package:

# ./rego/common.rego
# rego function to be used in other policies

package k8s.common  # Package definition that needs to be imported in other policies

import future.keywords 

valid_key(obj, key) {
	_ = obj[key]
	not is_null(obj[key])
} else = false {
	true
}