p2 proxy repository does not work with IQ update site

Create a p2 proxy repository with a remote URL of:

https://download.sonatype.com/clm/eclipse/releases

Attempt to install plugins through the proxy. This will fail.

Requesting the compositeContent.xml file through the proxy shows the location URLs in the metadata are not rewritten:

<?compositeMetadataRepository version='1.0.0'?>
<repository name="Sonatype CLM Repository" type="org.eclipse.equinox.internal.p2.metadata.repository.CompositeMetadataRepository" version="1.0.0">
<properties size="2">
<property name="p2.timestamp" value="1308747727547"/>
<property name="p2.compressed" value="false"/>
</properties>
<children>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.15.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.14.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.14.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.17.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.3"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.2"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.4.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.5.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.4"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.9.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.13.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.13.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.6.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.6.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.10.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.7.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.7.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.8.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.11.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.17.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.12.0"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.16.1"/>
<child location="https/download.sonatype.com/clm/eclipse/releases/2.16.0"/>
</children>
</repository>

And the request.log confirms this. The compositeContent.xml is downloaded by Eclipse:

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "GET /repository/aaa/compositeContent.xml HTTP/1.1" 200 - 2593 5 "p2/mars-sr0 (Java 1.8.0_222-b10 AdoptOpenJDK; MacOSX 10.15.4 x86-64; en_US) org.eclipse.sdk.ide/4.6.1.M20160907-1200 (org.eclipse.ui.ide.workbench)" [qtp1730016879-212]

And then Eclipse starts making requests as if the URL's in the XML file were a relative paths, rather than URLs. These requests all fail, and consequentially nothing can be installed in Eclipse. 

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "GET /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.15.0/p2.index HTTP/1.1" 404 - 1867 48 "p2/mars-sr0 (Java 1.8.0_222-b10 AdoptOpenJDK; MacOSX 10.15.4 x86-64; en_US) org.eclipse.sdk.ide/4.6.1.M20160907-1200 (org.eclipse.ui.ide.workbench)" [qtp1730016879-199]

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "HEAD /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.15.0/content.jar HTTP/1.1" 404 - 1867 54 "Apache-HttpClient/4.3.6 (java 1.5)" [qtp1730016879-196]

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "HEAD /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.15.0/content.xml HTTP/1.1" 404 - 1867 46 "Apache-HttpClient/4.3.6 (java 1.5)" [qtp1730016879-51]

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "HEAD /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.15.0/content.xml.xz HTTP/1.1" 404 - 1867 47 "Apache-HttpClient/4.3.6 (java 1.5)" [qtp1730016879-211]

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "HEAD /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.15.0/compositeContent.jar HTTP/1.1" 404 - 1867 2 "Apache-HttpClient/4.3.6 (java 1.5)" [qtp1730016879-212]

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "HEAD /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.15.0/compositeContent.xml HTTP/1.1" 404 - 1867 1 "Apache-HttpClient/4.3.6 (java 1.5)" [qtp1730016879-199]

127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "GET /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.15.0/site.xml HTTP/1.1" 404 - 1867 1 "p2/mars-sr0 (Java 1.8.0_222-b10 AdoptOpenJDK; MacOSX 10.15.4 x86-64; en_US) org.eclipse.sdk.ide/4.6.1.M20160907-1200 (org.eclipse.ui.ide.workbench)" [qtp1730016879-196]
127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "GET /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.14.1/p2.index HTTP/1.1" 404 - 1867 47 "p2/mars-sr0 (Java 1.8.0_222-b10 AdoptOpenJDK; MacOSX 10.15.4 x86-64; en_US) org.eclipse.sdk.ide/4.6.1.M20160907-1200 (org.eclipse.ui.ide.workbench)" [qtp1730016879-51]
127.0.0.1 - - [15/Apr/2020:10:35:12 -0500] "HEAD /repository/aaa/https/download.sonatype.com/clm/eclipse/releases/2.14.1/content.jar HTTP/1.1" 404 - 1867 54 "Apache-HttpClient/4.3.6 (java 1.5)" [qtp1730016879-211]

 h4. Expected

Make common P2 proxy sites work.