Cache loaded repos in ProvisioningContext

Currently if fetching updates the provision context is queried multiple
times leading to the repositories loaded multiple times, even though
this is usually faster as repo are cached this still produces
unnecessary workload, especially if there is one failed repository it
will be tried to be loaded over and over again.

This now caches the repositories loaded on first access to speed that
up.

bundles/org.eclipse.equinox.p2.engine/src/org/eclipse/equinox/p2/engine/ProvisioningContext.java

@@ -223,18 +223,22 @@ private void getLoadedRepository(URI location, IArtifactRepositoryManager repoMa
 	}
 
 	private Set<IMetadataRepository> getLoadedMetadataRepositories(IProgressMonitor monitor) {
-		IMetadataRepositoryManager repoManager = agent.getService(IMetadataRepositoryManager.class);
-		URI[] repositories = metadataRepositories == null ? repoManager.getKnownRepositories(IRepositoryManager.REPOSITORIES_ALL) : metadataRepositories;
-
-		Map<String, IMetadataRepository> repos = new HashMap<>();
-		SubMonitor sub = SubMonitor.convert(monitor, repositories.length);
-
-		// Clear out the list of remembered artifact repositories
-		referencedArtifactRepositories = new HashMap<>();
-		for (URI repositorie : repositories) {
-			loadMetadataRepository(repoManager, repositorie, repos, shouldFollowReferences(), sub.split(1));
+		if (loadedRepos == null) {
+			IMetadataRepositoryManager repoManager = agent.getService(IMetadataRepositoryManager.class);
+			URI[] repositories = metadataRepositories == null
+					? repoManager.getKnownRepositories(IRepositoryManager.REPOSITORIES_ALL)
+					: metadataRepositories;
+
+			loadedRepos = new HashMap<>();
+			SubMonitor sub = SubMonitor.convert(monitor, repositories.length);
+
+			// Clear out the list of remembered artifact repositories
+			referencedArtifactRepositories = new HashMap<>();
+			for (URI repositorie : repositories) {
+				loadMetadataRepository(repoManager, repositorie, loadedRepos, shouldFollowReferences(), sub.split(1));
+			}
 		}
-		return new HashSet<>(repos.values());
+		return new HashSet<>(loadedRepos.values());
 	}
 
 	private void loadMetadataRepository(IMetadataRepositoryManager manager, URI location,
@@ -421,6 +425,7 @@ private <T, R extends IRepository<T>> void loadRepository(IRepositoryManager<T>
 	private static final Comparator<IArtifactKey> ARTIFACT_KEY_COMPARATOR = Comparator //
 			.comparing(IArtifactKey::getId) //
 			.thenComparing(IArtifactKey::getVersion);
+	private Map<String, IMetadataRepository> loadedRepos;
 
 	/**
 	 * Returns a map from simple artifact repository location to a subset of the