Make snippet-extract take individual input/output files instead of a directory

[bitjammer]

This does not change the experience for package authors but allows for better integration with other tools that require sandboxing of file access.

rdar://103034070

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

[bitjammer]

@swift-ci Please test

[bitjammer]

Hm... ./bin/test succeeds here locally. Going to run again in case it's a bot issue.

[bitjammer]

@swift-ci Please test

[bitjammer]

@swift-ci Please test

[bitjammer]

@swift-ci Please test