Simplify get_pkg_names function

- Hoist non-file-reading logic outside of the file context manager
- Use a dict instead of a list for faster / more Pythonic lookups
- Use a set to simplify the add / append logic
- Move import sorting from `get_all_imports` to `get_pkg_names` for
  to account for set ordering. This change may also affect #89.
- Add a docstring

pipreqs/pipreqs.py

@@ -120,7 +120,7 @@ def get_all_imports(
     with open(join("stdlib"), "r") as f:
         data = [x.strip() for x in f.readlines()]
         data = [x for x in data if x not in py2_exclude] if py2 else data
-        return sorted(list(set(packages) - set(data)))
+        return list(set(packages) - set(data))
 
 
 def filter_line(l):
@@ -224,18 +224,24 @@ def get_import_local(imports, encoding=None):
 
 
 def get_pkg_names(pkgs):
-    result = []
+    """Get PyPI package names from a list of imports.
+
+    Args:
+        pkgs (List[str]): List of import names.
+
+    Returns:
+        List[str]: The corresponding PyPI package names.
+
+    """
+    result = set()
     with open(join("mapping"), "r") as f:
-        data = [x.strip().split(":") for x in f.readlines()]
-        for pkg in pkgs:
-            toappend = pkg
-            for item in data:
-                if item[0] == pkg:
-                    toappend = item[1]
-                    break
-            if toappend not in result:
-                result.append(toappend)
-    return result
+        data = dict(x.strip().split(":") for x in f)
+    for pkg in pkgs:
+        # Look up the mapped requirement. If a mapping isn't found,
+        # simply use the package name.
+        result.add(data.get(pkg, pkg))
+    # Return a sorted list for backward compatibility.
+    return sorted(result)
 
 
 def get_name_without_alias(name):