Fix performance regression in compiler

When the commit at
8c4a74a3ed
introduced a recursive search for build elements, it accidentally did so
using a function that accepts a regex for its match.

In doing so, if a behaviour or include had a name such as
`common_prefix` and that multiple other modules used the name
`common_prefix_<rest_of_name>`, all the other files would be seen as
dependencies to be checked in a directed graph.

This resulted in continuous re-checking of files that kept depending on
each others and blew the compile time up exponentially.

By using a delimitation on the regex (^<modulename>$), the build comes
back down to finding only the right files and becomes fast again.

src/rebar_compiler_erl.erl

@@ -317,7 +317,7 @@ expand_file_names(Files, Dirs) ->
                   true ->
                       [Incl];
                   false ->
-                      rebar_utils:find_files_in_dirs(Dirs, Incl, true)
+                      rebar_utils:find_files_in_dirs(Dirs, [$^, Incl, $$], true)
               end
       end, Files).