Update the libgit2 modulemap to avoid "does not include header" warnings

Also, use symlinks to enable cleaner path strings in the modulemap.

libgit2/git2

@@ -0,0 +1 @@
+../External/libgit2/include/git2

libgit2/git2.h

@@ -0,0 +1 @@
+../External/libgit2/include/git2.h

libgit2/module.modulemap

@@ -1,6 +1,21 @@
 module libgit2 {
-	umbrella header "../External/libgit2/include/git2.h"
+	umbrella header "git2.h"
 
-	export *
+	// Exclude headers intended only for Microsoft compilers
-	module * { export * }
+	exclude header "git2/inttypes.h"
+	exclude header "git2/stdint.h"
+
+	// Explicit modules for headers not included in the umbrella header:
+	explicit module cred_helpers {
+		header "git2/cred_helpers.h"
+	}
+
+	explicit module trace {
+		header "git2/trace.h"
+	}
+
+	// Explicit module for the "sys" headers:
+	explicit module sys {
+		umbrella "git2/sys"
+	}
 }