remove N-API implementation, v6.x and v8.x support

Remove the followings:

* the files associated with the external implementation of N-API
* Travis CI jobs for v8.x and v6.x
* documentation instructing users to add the external N-API
  implementation to their dependencies.
* conversion tool code that adds the external N-API implementation as a
  dependency to the user's addon.

This move is possible because of v8.x EOL, which means that all
supported versions of Node.js now have an internal implementation of
N-API.

Fixes: nodejs/node-addon-api#463
Fixes: nodejs/node-addon-api#509
Fixes: nodejs/node-addon-api#640
PR-URL: nodejs/node-addon-api#643
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

.travis.yml

@@ -12,8 +12,6 @@ env:
     # https://github.com/jasongin/nvs/blob/master/doc/CI.md
     - NVS_VERSION=1.4.2
   matrix:
-    - NODEJS_VERSION=node/6
-    - NODEJS_VERSION=node/8
     - NODEJS_VERSION=node/10
     - NODEJS_VERSION=node/12
     - NODEJS_VERSION=node/13
@@ -22,7 +20,6 @@ matrix:
   fast_finish: true
   allow_failures:
     - env: NODEJS_VERSION=nightly
-    - env: NODEJS_VERSION=node/6
 sudo: false
 cache:
   directories:

common.gypi

@@ -18,7 +18,6 @@
   ],
   'defines': ['NODE_MAJOR_VERSION=<@(NODE_MAJOR_VERSION)'],
   'include_dirs': ["<!@(node -p \"require('../').include\")"],
-  'dependencies': ["<!(node -p \"require('../').gyp\")"],
   'cflags': [ '-Werror', '-Wall', '-Wextra', '-Wpedantic', '-Wunused-parameter' ],
   'cflags_cc': [ '-Werror', '-Wall', '-Wextra', '-Wpedantic', '-Wunused-parameter' ]
 }

doc/setup.md

@@ -27,7 +27,6 @@ To use **N-API** in a native module:
 
 ```gyp
   'include_dirs': ["<!@(node -p \"require('node-addon-api').include\")"],
-  'dependencies': ["<!(node -p \"require('node-addon-api').gyp\")"],
 ```
 
   3. Decide whether the package will enable C++ exceptions in the N-API wrapper.

index.js

@@ -1,45 +1,8 @@
-var path = require('path');
-
-var versionArray = process.version
-  .substr(1)
-  .replace(/-.*$/, '')
-  .split('.')
-  .map(function(item) {
-    return +item;
-  });
-
-// TODO: Check if the main node semantic version is within multiple ranges,
-// or detect presence of built-in N-API by some other mechanism TBD.
-
-// We know which version of Node.js first shipped the incarnation of the API
-// available in *this* package. So, if we find that the Node.js version is below
-// that, we indicate that the API is missing from Node.js.
-var isNodeApiBuiltin = (
-  versionArray[0] > 8 ||
-  (versionArray[0] == 8 && versionArray[1] >= 6) ||
-  (versionArray[0] == 6 && versionArray[1] >= 15) ||
-  (versionArray[0] == 6 && versionArray[1] >= 14 && versionArray[2] >= 2));
-
-// The flag is not needed when the Node version is not 8, nor if the API is
-// built-in, because we removed the flag at the same time as creating the final
-// incarnation of the built-in API.
-var needsFlag = (!isNodeApiBuiltin && versionArray[0] == 8);
-
-var include = [__dirname];
-var gyp = path.join(__dirname, 'src', 'node_api.gyp');
-
-if (isNodeApiBuiltin) {
-  gyp += ':nothing';
-} else {
-  gyp += ':node-api';
-  include.unshift(path.join(__dirname, 'external-napi'));
-}
+const path = require('path');
 
 module.exports = {
-  include: include.map(function(item) {
-    return '"' + item + '"';
-  }).join(' '),
-  gyp: gyp,
-  isNodeApiBuiltin: isNodeApiBuiltin,
-  needsFlag: needsFlag
+  include: `"${__dirname}"`,
+  gyp: path.join(__dirname, 'nothing.gyp:nothing'),
+  isNodeApiBuiltin: true,
+  needsFlag: false
 };

node_api.gyp

@@ -0,0 +1,9 @@
+{
+  'targets': [
+    {
+      'target_name': 'nothing',
+      'type': 'static_library',
+      'sources': [ 'nothing.c' ]
+    }
+  ]
+}

package.json

@@ -211,6 +211,7 @@
     "safe-buffer": "^5.1.1"
   },
   "directories": {},
+  "gypfile": false,
   "homepage": "https://github.com/nodejs/node-addon-api",
   "keywords": [
     "n-api",