Allow to load dub.selections.json before Project instantiation

The aim of this commit is to have at least one constructor
for Project that does not perform IO. By moving the IO to the
`Dub` class, we can keep the dependency injection in `Dub`
and `PackageManager` only, and don't have to specialize `Project`.

source/dub/dub.d

@@ -456,7 +456,8 @@ class Dub {
 	/// Loads a specific package as the main project package (can be a sub package)
 	void loadPackage(Package pack)
 	{
-		m_project = new Project(m_packageManager, pack);
+		auto selections = Project.loadSelections(pack);
+		m_project = new Project(m_packageManager, pack, selections);
 	}
 
 	/** Loads a single file package.

source/dub/project.d

@@ -71,23 +71,54 @@ class Project {
 		this(package_manager, pack);
 	}
 
-	/// ditto
+	/// Ditto
 	this(PackageManager package_manager, Package pack)
+	{
+		auto selections = Project.loadSelections(pack);
+		this(package_manager, pack, selections);
+	}
+
+	/// ditto
+	this(PackageManager package_manager, Package pack, SelectedVersions selections)
 	{
 		m_packageManager = package_manager;
 		m_rootPackage = pack;
+		m_selections = selections;
+		reinit();
+	}
+
+	/**
+	 * Loads a project's `dub.selections.json` and returns it
+	 *
+	 * This function will load `dub.selections.json` from the path at which
+	 * `pack` is located, and returned the resulting `SelectedVersions`.
+	 * If no `dub.selections.json` is found, an empty `SelectedVersions`
+	 * is returned.
+	 *
+	 * Params:
+	 *	 pack = Package to load the selection file from.
+	 *
+	 * Returns:
+	 *	 Always a non-null instance.
+	 */
+	static package SelectedVersions loadSelections(in Package pack)
+	{
+		auto selverfile = (pack.path ~ SelectedVersions.defaultFile).toNativeString();
 
-		auto selverfile = (m_rootPackage.path ~ SelectedVersions.defaultFile).toNativeString();
-		if (existsFile(selverfile)) {
-			// TODO: Remove `StrictMode.Warn` after v1.40 release
-			// The default is to error, but as the previous parser wasn't
-			// complaining, we should first warn the user.
-			auto selected = parseConfigFileSimple!Selected(selverfile, StrictMode.Warn);
-			m_selections = !selected.isNull() ?
-				new SelectedVersions(selected.get()) : new SelectedVersions();
-		} else m_selections = new SelectedVersions;
+		// No file exists
+		if (!existsFile(selverfile))
+			return new SelectedVersions();
 
-		reinit();
+		// TODO: Remove `StrictMode.Warn` after v1.40 release
+		// The default is to error, but as the previous parser wasn't
+		// complaining, we should first warn the user.
+		auto selected = parseConfigFileSimple!Selected(selverfile, StrictMode.Warn);
+
+		// Parsing error, it will be displayed to the user
+		if (selected.isNull())
+			return new SelectedVersions();
+
+		return new SelectedVersions(selected.get());
 	}
 
 	/** List of all resolved dependencies.