static DirManager::Get()...

... not member functions of AudacityProject

This puts DirManager.cpp and four others back into the big strongly connected
component of link dependencies.  They will break out again when Project.cpp
becomes a low-level file.

src/MissingAliasFileDialog.cpp

@@ -108,7 +108,7 @@ namespace MissingAliasFilesDialog {
          size_t numProjects = gAudacityProjects.size();
          for (size_t ii = 0; ii < numProjects; ++ii) {
             // search each project for the blockfile
-            if (gAudacityProjects[ii]->GetDirManager()->ContainsBlockFile(b)) {
+            if (DirManager::Get( *gAudacityProjects[ii] ).ContainsBlockFile(b)) {
                m_LastMissingBlockFileProject = gAudacityProjects[ii];
                break;
             }