The P2 Update site is here:
https://repo1.maven.org/maven2/.m2e/connectors/m2eclipse-buildhelper/0.15.0/N/0.15.0.201207090124/
This answer helped me to discover how Eclipse knows it.
There is a Catalog that can be opened from the preferences:

You can follow the link manually: http://download.eclipse.org/technology/m2e/discovery/directory-1.8.xml
If you download the org.eclipse.m2e.discovery.oss-catalog-1.6.jar
referenced as entry and you unzip it, the you will find a plugin.xml
file:
org.eclipse.m2e.discovery.oss-catalog-1.6
├── META-INF
│ └── MANIFEST.MF
├── images
│ ├── extras-48.png
│ └── scm-32.png
├── lifecycle
│ ├── com.coderplus.m2e.jaxws.feature.group.pluginxml
│ ├── com.coderplus.m2e.jaxws.feature.group.xml
│ ├── ..
│ ├── org.eclipse.m2e.discovery.lifecycles.tycho.pluginxml
│ ├── org.eclipse.m2e.discovery.lifecycles.tycho.xml
│ ├── org.maven.ide.eclipse.ajdt.pluginxml
│ └── org.maven.ide.eclipse.ajdt.xml
└── plugin.xml
And this file contains the information with the P2 Update site:
<connectorDescriptor siteUrl="http://repo1.maven.org/maven2/.m2e/connectors/m2eclipse-buildhelper/0.15.0/N/0.15.0.201207090124/" id="org.eclipse.m2e.discovery.lifecyclemapping.buildhelper" groupId="lifecycles" description="buildhelper" categoryId="org.eclipse.m2e.discovery.category.lifecycles" name="buildhelper" provider="Takari, Inc." license="EPL" kind="lifecycles">
<iu id="org.sonatype.m2e.buildhelper.feature.feature.group" />
<overview summary="buildhelper" />
</connectorDescriptor>