Various regression tests fixed by ocaml/dune#992

Interaction of (menhir ...) and -p
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This used to fail because dune couldn't associate a compilation
context to the menhir files when package bar was hidden.

  $ (cd menhir-and-dash-p && dune build -p foo)

package field without public_name field
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This used to fail because the parser for the "package" field when
there is no "public_name"/"public_names" field used to not parse the
argument of "package".

  $ (cd package-without-pub-name && dune build -p foo)
  File "dune", line 3, characters 10-13:
  3 |  (package foo))
                ^^^
  Error: This field is useless without a (public_name ...) field.
  [1]
