Move gatsby-link into gatsby (#3864)

* move Link component from gatsby-link to gatsby package

This change is mostly copying the files from `gatsby-link` package to `gatsby` package. Have added a missing dependency (prop-types) and changed the `main` file to `dist/index.js` in gatsby's package.json.

* add TypeScript definition for Link component

Just copying over index.d.ts from `gatsby-link` package to `gatsby` and removing default export as we don't export `Link` component as the default one.

* delete babelrc from gatsby package

existing babel setup should already handle this!

* revert moving gatsby-link source to gatsby

* browser.js to export link components

packages/gatsby/package.json

@@ -146,6 +146,7 @@
   ],
   "license": "MIT",
   "main": "index.js",
+  "browser": "dist/browser.js",
   "repository": {
     "type": "git",
     "url": "git+https://github.com/gatsbyjs/gatsby.git"
@@ -160,4 +161,4 @@
     "test-coverage": "node_modules/.bin/nyc --reporter=lcov --reporter=text npm test",
     "watch": "rimraf dist && mkdir dist && npm run build:internal-plugins && npm run build:rawfiles && npm run build:src -- --watch"
   }
-}
+}

packages/gatsby/src/browser.js

@@ -0,0 +1,3 @@
+import Link, { withPrefix, navigateTo } from 'gatsby-link'
+
+export { Link, withPrefix, navigateTo }

yarn.lock

@@ -5298,22 +5298,6 @@ gatsby-remark-smartypants@canary:
     retext-smartypants "^2.0.0"
     unist-util-visit "^1.1.1"
 
-gatsby-source-filesystem@^1.5.15:
-  version "1.5.15"
-  resolved "https://registry.yarnpkg.com/gatsby-source-filesystem/-/gatsby-source-filesystem-1.5.15.tgz#26278e00101f03c279f4f0bdd285420ff561e8b2"
-  dependencies:
-    babel-cli "^6.26.0"
-    babel-runtime "^6.26.0"
-    bluebird "^3.5.0"
-    chokidar "^1.7.0"
-    fs-extra "^4.0.1"
-    got "^7.1.0"
-    md5-file "^3.1.1"
-    mime "^1.3.6"
-    pretty-bytes "^4.0.2"
-    slash "^1.0.0"
-    valid-url "^1.0.9"
-
 gatsby-source-filesystem@canary:
   version "2.0.0-alpha.3f307d61"
   resolved "https://registry.yarnpkg.com/gatsby-source-filesystem/-/gatsby-source-filesystem-2.0.0-alpha.3f307d61.tgz#3b57ce8e634cba2e2b997be1917bce28b951d235"
@@ -8733,7 +8717,7 @@ mime@1.4.1:
   version "1.4.1"
   resolved "https://registry.yarnpkg.com/mime/-/mime-1.4.1.tgz#121f9ebc49e3766f311a76e1fa1c8003c4b03aa6"
 
-mime@^1.2.11, mime@^1.3.4, mime@^1.3.6, mime@^1.4.1, mime@^1.5.0:
+mime@^1.2.11, mime@^1.3.4, mime@^1.4.1, mime@^1.5.0:
   version "1.6.0"
   resolved "https://registry.yarnpkg.com/mime/-/mime-1.6.0.tgz#32cd9e5c64553bd58d19a568af452acff04981b1"