Carve check_import_accessibility.m out of grab_modules.m.

The new module is completely independent of the rest of grab_modules.m.

compiler/check_import_accessibility.m:
compiler/grab_modules.m:
    As above.

    Make some slight changes in predicate names.

compiler/parse_tree.m:
compiler/notes/compiler_design.html:
    Include and document the new module.

compiler/notes/compiler_design.html

@@ -506,8 +506,12 @@ This is done by the following modules.
 
 <ul>
 <li>
-grab_modules.m figures out what interface files to read,
-and also does a bunch of other semi-related things.
+grab_modules.m figures out what interface files to read, and reads them.
+It also does a bunch of other semi-related things.
+<li>
+check_import_accessibility.m checks whether imported (and used) modules
+are actually accessible (by having imports of all their ancestor modules),
+and generates error messages if the answer is "no".
 <li>
 read_module.m has code to read in source, interface and optimization files.
 <li>

compiler/parse_tree.m

@@ -107,6 +107,7 @@
 % Transformations that act on the parse tree,
 % and stuff relating to the module system.
 :- include_module canonicalize_interface.
+:- include_module check_import_accessibility.
 :- include_module check_module_interface.
 :- include_module comp_unit_interface.
 :- include_module convert_parse_tree.