patch 8.2.4650: "import autoload" only works with using 'runtimepath'

Problem:    "import autoload" only works with using 'runtimepath'.
Solution:   Also support a relative and absolute file name.
diff --git a/src/vim9.h b/src/vim9.h
index 81ca147..345060a 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -28,6 +28,8 @@
     ISN_ECHOERR,    // :echoerr with isn_arg.number items on top of stack
     ISN_RANGE,	    // compute range from isn_arg.string, push to stack
     ISN_SUBSTITUTE, // :s command with expression
+
+    ISN_SOURCE,	    // source autoload script, isn_arg.number is the script ID
     ISN_INSTR,	    // instructions compiled from expression
 
     // get and set variables
@@ -43,6 +45,7 @@
     ISN_LOADWDICT,  // push w: dict
     ISN_LOADTDICT,  // push t: dict
     ISN_LOADS,	    // push s: variable isn_arg.loadstore
+    ISN_LOADEXPORT, // push exported variable isn_arg.loadstore
     ISN_LOADOUTER,  // push variable from outer scope isn_arg.outer
     ISN_LOADSCRIPT, // push script-local variable isn_arg.script.
     ISN_LOADOPT,    // push option isn_arg.string
@@ -57,6 +60,7 @@
     ISN_STOREW,	    // pop into window-local variable isn_arg.string
     ISN_STORET,	    // pop into tab-local variable isn_arg.string
     ISN_STORES,	    // pop into script variable isn_arg.loadstore
+    ISN_STOREEXPORT, // pop into exported script variable isn_arg.loadstore
     ISN_STOREOUTER,  // pop variable into outer scope isn_arg.outer
     ISN_STORESCRIPT, // pop into script variable isn_arg.script
     ISN_STOREOPT,    // pop into option isn_arg.storeopt