Build any unit as starting point

[zertovitch]

Instead of building a main procedure (and only that)
HAC will build any unit X as starting point using an implicit
"super main":

with X;

procedure _Super_Main is
begin
  || X;     --  if X is a parameterless procedure
  || null;  --  if X is another kind of unit
end;

The advantage is that HAC will be able to check semantics of any unit.

• hac.exe x.ad{s|b} can check any unit; you don't need to search
  for a main procedure WITH-ing that unit for that job.
• the Semantics target can check any unit as starting point;
  as a consequence an editor like LEA can refresh the analysis
  of the unit being worked on even if it is not a potential main
  procedure.

[zertovitch]

Compile_Main should be rearranged in order to call Compile_Unit instead of parsing an hypothetical main procedure.
It will remove code duplication between Compile_Main and Compile_Unit.
Perhaps the "super main" won't be needed in the end.

[zertovitch]

The 2nd way was taken and since commit dc346c8, any kind of unit can be the starting point, including with non-trivial file catalogues like LEA's.
In a nutshell, you can start a build with:

1. a subprogram spec (HAC will look for the body, among other bodies)
2. a subprogram body
3. a package spec (HAC will look for the body among other bodies)
4. a package body (HAC will restart the build with the spec as in point 3)