Don't decide a file's directory

Don't decide a file's directory
until the user actually tries to go there.
(compilation-next-error-function):
Pass compilation-find-file the directory from the file-struct.
(compilation-internal-error-properties): Separate local FILE-STRUCT
from FILE.  Doc the args better.  Rename arg FMT to FMTS.
(compilation-find-file): Arg DIR renamed to DIRECTORY.
Expand it, and if nil, use default-directory.
(compilation-get-file-structure): Don't mix specified directory
with default directory.  Put specified directory into
file-struct.  Don't make the file name absolute.

(compilation-error-regexp-alist): Doc fix.
(compile-command): Add autoload.
(compilation-disable-input): Add autoload.