1.0.27.46: Fix build on systems with "src" in the path.

1.0.27.46: Fix build on systems with "src" in the path.

  1.0.27.31 introduced an actual check that the path for all source
files is correctly externalized as an LPN at cold-init time.  Due to a
longstanding bug in MAKE-FILE-INFO-NAMESTRING, not fixed with
1.0.27.31, it is possible for the system to create a pathname such as
"SYS:SRC;LISP;SBCL;SBCL-GIT;OUTPUT;STUFF-GROVELED-FROM-HEADERS.LISP".
Once the SYS: logical pathname translations are set up, this path is
not valid, causing a build failure.  Fixed, at the cost of disallowing
paths in SYS:SRC that have a final directory of OUTPUT, not likely to
be an issue in practice.