Use display for external deps and document this new feature.

nimble.nimble

@@ -1,4 +1,4 @@
-import ospaths, distros
+import ospaths
 template thisModuleFile: string = instantiationInfo(fullPaths = true).filename
 
 when fileExists(thisModuleFile.parentDir / "src/nimblepkg/common.nim"):
@@ -22,10 +22,12 @@ srcDir = "src"
 
 requires "nim >= 0.13.0", "compiler#head"
 
-if detectOs(Ubuntu):
-  foreignDep "libssl-dev"
-elif detectOs(MacOSX):
-  foreignDep "openssl"
+when defined(nimdistros):
+  import distros
+  if detectOs(Ubuntu):
+    foreignDep "libssl-dev"
+  else:
+    foreignDep "openssl"
 
 task test, "Run the Nimble tester!":
   withDir "tests":