Build any unit as starting point

[zertovitch]

Build any unit as starting point

[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)