[build] Use `https` instead of `git` protocol in modules (#3848)

This allows contributors to init submodules without needing to have a
[an SSH keypair on their GitHub account][0].

[0]: https://help.github.com/en/github/using-git/which-remote-url-should-i-use#cloning-with-ssh-urls

.gitmodules

@@ -4,7 +4,7 @@
     branch = master
 [submodule "external/debugger-libs"]
     path = external/debugger-libs
-    url = git://github.com/mono/debugger-libs
+    url = https://github.com/mono/debugger-libs
     branch = master
 [submodule "external/dlfcn-win32"]
     path = external/dlfcn-win32
@@ -20,7 +20,7 @@
     branch = master
 [submodule "external/nrefactory"]
     path = external/nrefactory
-    url = git://github.com/icsharpcode/NRefactory.git
+    url = https://github.com/icsharpcode/NRefactory.git
     branch = master
 [submodule "external/opentk"]
     path = external/opentk