Omit the "doc://" and identifier prefix from V2 link(_:) requests

[d-ronnqvist]

Bug/issue #, if applicable: rdar://162694487

Summary

This makes a small simplification to the very recently introduced V2 external resolver communication protocol to omit the known "doc://" and identifier prefix—which is the same for every request—so that the resolver implementation doesn't need to parse it to get to the unique portion of the link request.

Dependencies

None.

Testing

• Modify the bin/test-data-external-resolver script to write the requests to some file by adding echo $REPLY >> /path/to/some-file after the read line
• Set the path to the bin/test-data-external-resolver script for the DOCC_LINK_RESOLVER_EXECUTABLE environmental variable
• Modify some content to add one or more external links that use the "com.test.bundle" identifier. For example <doc://com.test.bundle/path/to/something>
• Build documentation for that content
• Check the contents of /path/to/some-file
  • Each raw request string should only include the path (and fragment where applicable) of the link string.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

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

[d-ronnqvist]

@swift-ci please test

[anferbui]

Looks good to me with a very minor comment

[d-ronnqvist]

@swift-ci please test