[xbuild] Add env var to dump details of reference resolution.

[xbuild] Add env var to dump details of reference resolution.

If xbuild is unable to resolve a reference, then it dumps details of
why it failed. This adds a env var XBUILD_LOG_REFERENCE_RESOLVER,
which if set, logs those details of why a particular search path
was used to resolve the reference.

Update the man page also.