fix for using @ in conanfile.txt imports

[memsharded]

Changelog: Bugfix: Fixed problem with conanfile.txt [imports] sections using the '@' character.
Docs: conan-io/docs#1078

Close #4524

There was a problem with folders using "@" and not extra arguments being specified, it failed because the rsplit(, 1) was splitting it. Possibilities:

• Use spaces <blank>@<blank> to split path<->args. Could break some users if they put @folders=True. But simple, will work without needing a trailing @.
• Force users with @ in their pathnames to define a trailing @, even if empty (implemented solution).
• Do a full fledged parser that regexs over the folders=True, etc. patterns (too complex for the value).