Uses a single code path to link and unlink all platforms

Summary:
This commit removes special cases for linking iOS and Android platforms.

A previous commit opened up link and other commands for other platforms to provide their own behaviors. It left special cases in tact for iOS and Android. This PR removes the special case.

- Added jest tests related to the link command.
- Ran the `link` and `unlink` commands for iOS and Android and confirmed no changes.

https://github.com/facebook/react-native/pull/17745

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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAL  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
[ ANDROID  ]   [ MINOR       ]   [ {Framework} ] - | {Message} |
[----------]   [-------------]   [-------------]   |-----------|

[CATEGORY] [TYPE] [LOCATION] - MESSAGE

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->

[CLI][FEATURE][local-cli/link/link.js] - Removes special cases for linking in iOS and Android.
Closes https://github.com/facebook/react-native/pull/17961

Differential Revision: D6975951

Pulled By: hramos

fbshipit-source-id: 8dd5da35619e2124ce4b3b18db8b694757792363

local-cli/core/android/index.js

@@ -126,3 +126,5 @@ exports.dependencyConfig = function dependencyConfigAndroid(folder, userConfig)
 
   return { sourceDir, folder, manifest, packageImportPath, packageInstance };
 };
+
+exports.linkConfig = require('../../link/android');

local-cli/core/ios/index.js

@@ -57,3 +57,5 @@ exports.projectConfig = function projectConfigIOS(folder, userConfig) {
 };
 
 exports.dependencyConfig = exports.projectConfig;
+
+exports.linkConfig = require('../../link/ios');