Rename function in completion.el to use completion- prefixes

* lisp/completion.el (locate-completion-entry)
(completion-locate-entry, locate-completion-entry-retry)
(completion-locate-entry-retry, locate-completion-db-error):
Rename to use a completion- prefix and make the old names obsolete
aliases.
(add-completion-to-head, delete-completion): Use the new function
names.
1 job for master in 90 minutes and 2 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #2283
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds