Improve support for non-standalone XSLT stylesheet modules

[pgfearo]

Currently, when non-standalone XSLT stylesheet modules there may be issues due to 'missing' imports that are actually provided by a parent stylesheet module. The potential issues are:

• Problems reported for references to undeclared symbols like variables names or function calls
• No auto-completion for symbols that are available in the 'missing' imports
• No 'Goto Definition' for references to symbols that are defined in the 'missing' imports

[pgfearo]

Completed