Move check for --extra and --all-extras outside loop

Previously, an error would always be thrown when running `compile` with
`--all-extras` on multiple source files containing extras. The reason was
that the first iteration over the first source file would fill the
`extras` variable, which in the second iteration would trigger an error
since both `all_extras` and `extras` would be set.

This change moves the check outside the loop and early in the script to
avoid unnecessary processing before throwing the error.

piptools/scripts/compile.py

@@ -174,6 +174,10 @@ def cli(
                 ).format(", ".join(DEFAULT_REQUIREMENTS_FILES))
             )
 
+    if all_extras and extras:
+        msg = "--extra has no effect when used with --all-extras"
+        raise click.BadParameter(msg)
+
     if not output_file:
         # An output file must be provided for stdin
         if src_files == ("-",):
@@ -336,10 +340,7 @@ def cli(
             )
 
             if all_extras:
-                if extras:
-                    msg = "--extra has no effect when used with --all-extras"
-                    raise click.BadParameter(msg)
-                extras = tuple(metadata.get_all("Provides-Extra"))
+                extras += tuple(metadata.get_all("Provides-Extra"))
         else:
             constraints.extend(
                 parse_requirements(