Enhanced error messages for invalid library/executables names

[voodoos]

Actual behavior

When an executable has an invalid name, dune wrongly states that this module doesn't exist, while it not a valid module name:

File "dune", line 2, characters 7-16:
2 |  (name main-test)
           ^^^^^^^^^
Error: Module "Main-test" doesn't exist.

Moreover libraries have a better message when only the public_name is specified and it is not a valid module name:

Public library names don't have this restriction. You
  can either change this public name to be a valid library
  name or add a \"name\" field with a valid library name."

This is not the case for executables.

In this PR

• The first two commits implement the same error granularity for executables than for libraries, as described hereinabove.
• The last one adds an hint to suggest a valid name to the user.

New behavior (see tests for more)

  File "dune", line 2, characters 7-10:
  2 |  (name a.b))
             ^^^
  Error: "a.b" is an invalid module name.
  Module names must be non-empty and composed only of the following characters:
  'A'..'Z', 'a'..'z', '_', ''' or '0'..'9'.
  Hint: a_b would be a correct module name

  File "dune", line 2, characters 14-17:
  2 |  (public_name a.b))
                    ^^^
  Error: Invalid module name.
  Public executable names don't have this restriction. You can either change
  this public name to be a valid module name or add a "name" field with a valid
  module name.
  Hint: Module names must be non-empty and composed only of the following
  characters: 'A'..'Z', 'a'..'z', '_', ''' or '0'..'9'.
  Hint: a_b would be a correct module name

Note

These messages and validation do not take into account the fact that the ocamlc compiler does accept, with a warning, an invalid name for the main module:

$ ocamlc a-b.ml -o a-b
File "a-b.ml", line 1:
Warning 24: bad source file name: "A-b" is not a valid module name.

$ ./a-b 
hello world

[rgrinberg]

Can we specify the type of the value we're ignoring? That's a convention we follow in dune because it's error prone to keep ignoring values if the type returned changes.