[Fleet] Use type-only imports where possible (#92979)

## Summary

Use [type-only `import`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-8.html#type-only-imports-and-export) to differentiate between runtime values and (erasable) TS types.

<img width="1499" alt="Screen Shot 2021-02-27 at 6 34 46 PM" src="https://user-images.githubusercontent.com/57655/109403323-84e32c80-792a-11eb-94b7-854d0e5c3e99.png">

Allows Babel and `tsc` to make some optimizations since it can be sure what can be erased.

It can also be helpful in situations like below which highlights that even though `KibanaAssetType` and `KibanaSavedObjectType` are named a certain way and from `common/types`, they are JS values _not_ TS types. They are `enum`s which means they are JS objects at runtime; not eliminated at runtime.

https://github.com/elastic/kibana/blob/d92a1a08d89accc2fa303b1355a9e222f9b6d090/x-pack/plugins/fleet/common/types/models/epm.ts#L41-L63

Author: John Schulz

x-pack/plugins/fleet/common/constants/agent_policy.ts

@@ -6,7 +6,8 @@
  */
 
 import { defaultPackages } from './epm';
-import { AgentPolicy } from '../types';
+import type { AgentPolicy } from '../types';
+
 export const AGENT_POLICY_SAVED_OBJECT_TYPE = 'ingest-agent-policies';
 export const AGENT_POLICY_INDEX = '.fleet-policies';
 export const agentPolicyStatuses = {

x-pack/plugins/fleet/common/constants/output.ts

@@ -5,7 +5,7 @@
  * 2.0.
  */
 
-import { NewOutput } from '../types';
+import type { NewOutput } from '../types';
 
 export const OUTPUT_SAVED_OBJECT_TYPE = 'ingest-outputs';
 

x-pack/plugins/fleet/common/mocks.ts

@@ -5,7 +5,7 @@
  * 2.0.
  */
 
-import { NewPackagePolicy, PackagePolicy } from './types';
+import type { NewPackagePolicy, PackagePolicy } from './types';
 
 export const createNewPackagePolicyMock = (): NewPackagePolicy => {
   return {

x-pack/plugins/fleet/common/services/agent_status.ts

@@ -6,7 +6,7 @@
  */
 
 import { AGENT_POLLING_THRESHOLD_MS, AGENT_SAVED_OBJECT_TYPE } from '../constants';
-import { Agent, AgentStatus } from '../types';
+import type { Agent, AgentStatus } from '../types';
 
 export function getAgentStatus(agent: Agent, now: number = Date.now()): AgentStatus {
   const { last_checkin: lastCheckIn } = agent;

x-pack/plugins/fleet/common/services/full_agent_policy_kibana_config.ts

@@ -5,7 +5,7 @@
  * 2.0.
  */
 
-import { FullAgentPolicyKibanaConfig } from '../types';
+import type { FullAgentPolicyKibanaConfig } from '../types';
 
 export function getFullAgentPolicyKibanaConfig(kibanaUrls: string[]): FullAgentPolicyKibanaConfig {
   // paths and protocol are validated to be the same for all urls, so use the first to get them

x-pack/plugins/fleet/common/services/full_agent_policy_to_yaml.ts

@@ -6,7 +6,7 @@
  */
 
 import { safeDump } from 'js-yaml';
-import { FullAgentPolicy } from '../types';
+import type { FullAgentPolicy } from '../types';
 
 const POLICY_KEYS_ORDER = [
   'id',

x-pack/plugins/fleet/common/services/is_agent_upgradeable.test.ts

@@ -6,7 +6,7 @@
  */
 
 import { isAgentUpgradeable } from './is_agent_upgradeable';
-import { Agent } from '../types/models/agent';
+import type { Agent } from '../types/models/agent';
 
 const getAgent = ({
   version,

x-pack/plugins/fleet/common/services/is_agent_upgradeable.ts

@@ -7,7 +7,7 @@
 
 import semverCoerce from 'semver/functions/coerce';
 import semverLt from 'semver/functions/lt';
-import { Agent } from '../types';
+import type { Agent } from '../types';
 
 export function isAgentUpgradeable(agent: Agent, kibanaVersion: string) {
   let agentVersion: string;

x-pack/plugins/fleet/common/services/license.ts

@@ -6,7 +6,7 @@
  */
 
 import { Observable, Subscription } from 'rxjs';
-import { ILicense } from '../../../licensing/common/types';
+import type { ILicense } from '../../../licensing/common/types';
 
 // Generic license service class that works with the license observable
 // Both server and client plugins instancates a singleton version of this class

x-pack/plugins/fleet/common/services/limited_package.ts

@@ -5,7 +5,7 @@
  * 2.0.
  */
 
-import { PackageInfo, AgentPolicy, PackagePolicy } from '../types';
+import type { PackageInfo, AgentPolicy, PackagePolicy } from '../types';
 
 // Assume packages only ever include 1 config template for now
 export const isPackageLimited = (packageInfo: PackageInfo): boolean => {