Filter by prefix on no exact match (#442)

* Filter by prefix on no exact match

* Clarify comment

Author: nmattia

src/Niv/Cli.hs

@@ -386,11 +386,11 @@ filterPackages :: Sources -> Maybe PackagePattern -> Sources
 filterPackages (unSources -> sources) mPat = Sources $ case mPat of
   -- no pattern: return all packages
   Nothing -> sources
-  -- pattern (filter) provided: match exact
+  -- pattern (filter) provided: return an exact match; if no exact match, match by prefix
   Just (PackagePattern pat) -> case HMS.lookup (PackageName pat) sources of
     Just exact -> HMS.singleton (PackageName pat) exact
     Nothing ->
-      HMS.filterWithKey (\k _ -> unPackageName k == pat) sources
+      HMS.filterWithKey (\k _ -> pat `T.isPrefixOf` (unPackageName k)) sources
 
 parseCmdUpdate :: Opts.ParserInfo (NIO ())
 parseCmdUpdate =