Update Eclipse setup files

Update the eclipse setup files and contributing instructions for
compatibility with the latest Eclipse 2019-03 release. This commit
reverts the use the HTTPS in the `.setup` file namespace (since it
breaks the setup editor). The `xsi:schemaLocation` section still
uses HTTPS for actual XSD resolution.

See gh-16245

CONTRIBUTING.adoc

@@ -170,7 +170,7 @@ provision a new environment. To use the installer:
 * Select "`Eclipse IDE for Java Developers`" under "`Eclipse.org`" as the product to
   install and click "`next`".
 * For the "`Project`" click on "`+`" to add a new setup file. Select "`Github Projects`"
-  and browser for `<checkout>/eclipse/spring-boot-project.setup` from your locally cloned
+  and browse for `<checkout>/eclipse/spring-boot-project.setup` from your locally cloned
   copy of the source code. Click "`OK`" to add the setup file to the list.
 * Double-click on "`Spring Boot`" from the project list to add it to the list that will
   be provisioned then click "`Next`".
@@ -183,12 +183,20 @@ Once complete you should find that a local workspace has been provisioned comple
 all required Eclipse plugins. Projects will be grouped into working-sets to make the code
 easier to navigate.
 
+If you want to work on the `spring-boot-gradle-plugin` you should remove the imported Maven
+project and reimport it as a Gradle project.
+
+TIP: If you see import errors with `com.sun` packages make sure you have setup a valid
+`JavaSE-1.8` environment. From preferences select "`Java`", "`Installed JREs`",
+"`Execution Environments`" and make sure "`JavaSE-1.8`" points to a Java 1.8
+install (we use AdoptOpenJDK on our CI).
+
 
 
 ==== Manual Installation with M2Eclipse
 If you prefer to install Eclipse yourself you should use the
 https://eclipse.org/m2e/[M2Eclipse] eclipse plugin. If you don't already have m2eclipse
-installed it is available from the "Eclipse marketplace".
+installed it is available from the "`Eclipse marketplace`".
 
 Spring Boot includes project specific source formatting settings, in order to have these
 work with m2eclipse, we provide an additional Eclipse plugin that you can install:

eclipse/eclipse.properties

+copyright-year=2012-2019

eclipse/spring-boot-project.setup

 <?xml version="1.0" encoding="UTF-8"?>
 <setup:Project
     xmi:version="2.0"
-    xmlns:xmi="https://www.omg.org/XMI"
+    xmlns:xmi="http://www.omg.org/XMI"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns:jdt="http://www.eclipse.org/oomph/setup/jdt/1.0"
     xmlns:maven="http://www.eclipse.org/oomph/setup/maven/1.0"
     xmlns:predicates="http://www.eclipse.org/oomph/predicates/1.0"
-    xmlns:setup="https://www.eclipse.org/oomph/setup/1.0"
-    xmlns:setup.p2="https://www.eclipse.org/oomph/setup/p2/1.0"
+    xmlns:setup="http://www.eclipse.org/oomph/setup/1.0"
+    xmlns:setup.p2="http://www.eclipse.org/oomph/setup/p2/1.0"
     xmlns:setup.workingsets="http://www.eclipse.org/oomph/setup/workingsets/1.0"
     xmlns:workingsets="http://www.eclipse.org/oomph/workingsets/1.0"
-    xsi:schemaLocation="http://www.eclipse.org/oomph/setup/jdt/1.0 http://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/JDT.ecore http://www.eclipse.org/oomph/setup/maven/1.0 http://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/Maven.ecore http://www.eclipse.org/oomph/predicates/1.0 https://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/Predicates.ecore http://www.eclipse.org/oomph/setup/workingsets/1.0 http://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/SetupWorkingSets.ecore http://www.eclipse.org/oomph/workingsets/1.0 https://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/WorkingSets.ecore"
+    xsi:schemaLocation="http://www.eclipse.org/oomph/setup/jdt/1.0 https://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/JDT.ecore http://www.eclipse.org/oomph/setup/maven/1.0 https://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/Maven.ecore http://www.eclipse.org/oomph/predicates/1.0 https://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/Predicates.ecore http://www.eclipse.org/oomph/setup/workingsets/1.0 https://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/SetupWorkingSets.ecore http://www.eclipse.org/oomph/workingsets/1.0 https://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/WorkingSets.ecore"
     name="spring.boot.1.5.x"
     label="Spring Boot 1.5.x">
   <setupTask