Warn/error when alias have invalid characters

[samoht]

For instance, something like this will make dune explodes:

(alias
 ((name foo/bar)
  (action (echo toto))))

(alias
 ((name bar)
  (action (echo toto))))

$ jbuilder build  --root . @bar
File "src/build_system.ml", line 226, characters 4-10: Assertion failed
Backtrace:
Raised at file "src/build_system.ml", line 226, characters 4-43
Called from file "src/gen_rules.ml", line 913, characters 16-49
Called from file "src/gen_rules.ml", line 963, characters 8-37
Called from file "src/stdune/list.ml", line 13, characters 10-13
Called from file "src/gen_rules.ml", line 954, characters 4-773
Called from file "src/gen_rules.ml", line 993, characters 19-30
Called from file "src/build_system.ml", line 860, characters 6-62
Called from file "src/build_system.ml", line 836, characters 6-59
Re-raised at file "src/build_system.ml", line 847, characters 6-17
Called from file "src/build_system.ml" (inlined), line 804, characters 32-63
Called from file "src/build_system.ml", line 814, characters 4-24
Called from file "src/build_interpret.ml", line 101, characters 24-40
Called from file "src/build_interpret.ml", line 60, characters 31-43
Called from file "src/build_interpret.ml", line 60, characters 31-43
Called from file "src/build_system.ml", line 1184, characters 10-108
Called from file "src/fiber/fiber.ml", line 359, characters 6-13

And

$ jbuilder build  --root . @foo/bar
From the command line:
Error: Don't know about directory foo!

[samoht]

(note: allowing / in alias name could be a useful feature too)

[ghost]

That's not possible: aliases are per-directory