fix: implicit imports drop std/ prefix (#25780)

Preserves implicit imports instead of always storing the resolved
absolute filename. That lets the later StdPrefix warning check see the
original std/objectdollar spelling.

This is for situations where in cfg or cli warnings are enabled for the
prefix. Essentially a niche combination of compiler switches don't get
along e.g.

`-d:nimPreviewSlimSystem --warning:StdPrefix:on
--warningAsError:StdPrefix:on --import:std/objectdollar`

will cause:

`Error: objectdollar needs the 'std' prefix [StdPrefix]`

tests/compiler/tcmdline_import_std_prefix.nim

@@ -0,0 +1,10 @@
+discard """
+  matrix: "-d:nimPreviewSlimSystem --warning:StdPrefix:on --warningAsError:StdPrefix:on --import:std/objectdollar"
+  output: "(a: 23, b: 45)"
+"""
+
+type Foo = object
+  a, b: int
+
+let x = Foo(a: 23, b: 45)
+echo x