Symbolgraph extension symbols

[johnfairh]

Support the opt-in symbolgraph extension symbols for Swift 5.9.

These are for extensions from other modules only, extensions in the same module don't get these symbols. I've turned on the option for the test case because it runs on Swift 5.9 and exercises all the new paths -- looks good, grabs a bunch more information for those extensions including doc comments.

[johnfairh]

This all seems to work well.

Jazzy itself needs to support pre-5.9 swift so it does not set -emit-extension-block-symbols when asked to generate symbolgraphs itself. The user can override this if necessary as done in the test runner - we could be smarter I suppose.

Also fixes a couple of bugs:

• typo prevented full declaration being propgated
• set the module name correctly everywhere

Fixes #1368