8159248: ModuleFinder.of spec + 8158456: ModuleDescriptor.read does not verify dependence on java.base

[Alan Bateman]

I need a Reviewer for a patch is to address two conformance issues.

One is that the ModuleFinder.of(Path...) javadoc doesn't make it clear 
that an exception is thrown when a module descriptor cannot be created 
for an automatic module. As part of this I've changed the implementation 
to not silently skip services configuration files or contents that don't 
parse as identifiers. Also a small change to ModuleDescriptor.Builder in 
preparation for other changes.

The other issue is that the requires table in the Module attribute for 
java.base should be empty and should have at least an entry for 
java.base for other modules.

The changes are simple, most of the changes are extending the test coverage:

http://cr.openjdk.java.net/~alanb/8159248%2b8158456/webrev/

-Alan