Copy snippet symbol graphs to the correct final destination

[bitjammer]

The snippet symbol graph is currently being generated with the changes in e013865138b, however they are not being copied to the correct final destination.

The snippet-extract tool now takes input and output files instead of directories, however there is still one place in the plugin where we are still treating the output like a directory, so the snippet symbol graph basename path component looks like a directory, and needs additional basenames to put it in the right place with the correct file extension.

rdar://104045850

• [x] Added tests
• [x] Ran the ./bin/test script and it succeeded
• [x] Updated documentation if necessary

[bitjammer]

Thank you @ethan-kusters for the patch!

[bitjammer]

@swift-ci Please test

[ethan-kusters]

@swift-ci please test