Changed generation of PrivateImplementationDetails to append module name only when dealing with netmudules.

The goal of module name apending is to avoid clashes when combining multiple netmodules into multifile assembly. When building a regular assembly, appending module name is not serving any purpose and just causes unnecessary metadata differences.

Also in this change - when we do apend the module name, replace '.' with '_' when that happens. For example when we build a netmodule and its name is Foo.Bar.dll
More complicated name mangling schemes were discussed, but at this point we will do a simple '.' --> '_' as the least destabilizing change which is still sufficient in the most common case of having dots in the module name.

Fixes #1430