Use "builtin type" to locate "mcfly"

The command "which" is a non-POSIX external command, and technically,
its existence in the system is not ensured.  We here use Bash's
built-in command "type", which is the most reliable.  We prefix
"builtin" to "type" because some users overwrite "type" with a shell
function or an alias to mimic Windows' "type" command.

The use of the which command was first removed in Ref. [1].  However,
this was reverted after the issue in Ref. [2].  As far as a relative
path is not contained in PATH, the problem reported in Ref. [2] does
not seem to be reproducible in all the versions from Bash 3.0 to 5.2
and in the devel branch of Bash.  However, when the path "./bin" is
included in PATH, the "command -v" produces the relative path
"./bin/mcfly" as reported.  This seems to have been solved by using
the "which command in the reporter's environment, but the behavior of
the "which" command may depend on the implementation.  We should
explicitly resolve the relative path if the obtained MCFLY_PATH is
relative.

References:

[1] cantino#216
[2] cantino#292
[3] cantino#430 (comment)

mcfly.bash

@@ -28,6 +28,11 @@ function mcfly_initialize {
 
   # Find the binary
   MCFLY_PATH=${MCFLY_PATH:-$(builtin type -P mcfly)}
+  if [[ $MCFLY_PATH != /* ]]; then
+    # When the user include a relative path in PATH, "builtin type -P" may
+    # produce a relative path.  We convert relative paths to the absolute ones.
+    MCFLY_PATH=$PWD/$MCFLY_PATH
+  fi
   if [[ -z $MCFLY_PATH ]]; then
     echo "Cannot find the mcfly binary, please make sure that mcfly is in your path before sourcing mcfly.bash."
     return 1