Add support for dynamic extension dependencies

Suppose Module A is loaded through its pre-compiled version
Suppose Module B is loaded through its pre-compiled version
Suppose Module A depends on Module B

If the csproj of Module B is updated, Module B will be dynamically compiled.
We have to ensure Module A is also dynamically compiled, because Module A can't
reference the pre-compiled version of B anymore.

The way we enforce this is by asking every loader if they are compatible
with all module references of a module before picking it as the actual loader.
If a loader decides it's not compatible with the module references, the next
loader in order of priority will be considered.

--HG--
branch : dev

src/Orchard.Tests/Environment/Extensions/ExtensionLoaderCoordinatorTests.cs

@@ -69,6 +69,10 @@ namespace Orchard.Tests.Environment.Extensions {
                 throw new NotImplementedException();
             }
 
+            public bool IsCompatibleWithReferences(ExtensionDescriptor extension, IEnumerable<ExtensionProbeEntry> references) {
+                throw new NotImplementedException();
+            }
+
             public ExtensionProbeEntry Probe(ExtensionDescriptor descriptor) {
                 return new ExtensionProbeEntry { Descriptor = descriptor, Loader = this };
             }

src/Orchard.Tests/Environment/Extensions/ExtensionManagerTests.cs

@@ -68,6 +68,10 @@ namespace Orchard.Tests.Environment.Extensions {
                 throw new NotImplementedException();
             }
 
+            public bool IsCompatibleWithReferences(ExtensionDescriptor extension, IEnumerable<ExtensionProbeEntry> references) {
+                throw new NotImplementedException();
+            }
+
             public ExtensionProbeEntry Probe(ExtensionDescriptor descriptor) {
                 return new ExtensionProbeEntry { Descriptor = descriptor, Loader = this };
             }