Have packageFiles return empty list for packages w/o files

In particular, packages defined in startup.m2 (currently just User),
stdio, or currentString.

This was also causing a problem with "check User", which was running
into this issue via "needsPackage", so we add a unit test

Author: d-torrance

M2/Macaulay2/m2/packages.m2

@@ -553,10 +553,15 @@ debug GlobalDictionary := dict -> (
     checkShadow())
 
 packageFiles = pkg -> (
-    pkgaux := (
+    srcfile := realpath pkg#"source file";
+    if (
+	srcfile == "stdio"         or
+	srcfile == "currentString" or
+	match("/startup\\.m2(?:\\.in)?$", srcfile))
+    then {}
+    else prepend(srcfile,
 	if not pkg#?"auxiliary files" then {}
-	else select(values loadedFiles, match_(pkg#"auxiliary files")));
-    prepend(realpath pkg#"source file", pkgaux))
+	else select(values loadedFiles, match_(pkg#"auxiliary files"))))
 
 locate Package := pkg -> NumberedVerticalList (
     -- TODO: somehow keep track of the number of lines of each file

M2/Macaulay2/tests/normal/testing.m2

@@ -21,6 +21,9 @@ expectedCode = DIV{
 assert BinaryOperation(symbol ===, code pkgtest, expectedCode)
 assert BinaryOperation(symbol ===, code 0, expectedCode)
 
+TEST "assert Equation(1 + 1, 2)"
+check User
+
 assert( (for i from 1 to 2 list { for j from 1 to 2 do { if j == 2 then break 444; } }) === {{444}, {444}} ) -- see issue #2522