CMake was previously doing this itself before 4.0, but seems to be
inserting `/usr/bin/*` now. Resolve the `/usr/bin` trampoline ourselves
in a similar fashion to swiftly (but with xcrun).
Resolves rdar://163462990.
When `DYLD_(FRAMEWORK|LIBRARY)_PATH` is set, `dlopen` will first check
if the basename of the provided path is within any of its search paths.
Thus it's possible that only a single library is loaded for each
toolchain, rather than a separate like we expect. The paths should be
equal in this case, since the client plugin is loaded based on the path
of `sourcekitd.framework` (and we should only have one for the same
reason). Allow this case and just avoid re-initializing.
There is only one real class that implements the `SourceKitD` protocol, so there really isn’t any need for the protocol + class split at all. Unify them to make code simpler to reason about.
