Ensure that P2 performs all downloads over HTTPS

Rather than using a local composite repository that would result in P2
using HTTP mirrors, this commit switches to a non-composite repository
accessed over HTTPS and disables the use of mirrors to ensure that
all downloads use the specified repository and access it over HTTPS.

Author: wilkinsona

eclipse-repository/compositeContent.jar

@@ -31,7 +31,8 @@
 		<main.basedir>${basedir}</main.basedir>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<java.version>1.8</java.version>
-		<eclipse.repository>file:${maven.multiModuleProjectDirectory}/eclipse-repository</eclipse.repository>
+		<eclipse.repository>https://download.eclipse.org/releases/oxygen/201804111000</eclipse.repository>
+		<tycho.disableP2Mirrors>true</tycho.disableP2Mirrors>
 		<ant.version>1.8.1</ant.version>
 		<ant-contrib.version>1.0b3</ant-contrib.version>
 		<asm.version>5.2</asm.version>