fix: implicit imports drop std/ prefix

[Graveflo]

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]

[Araq]

Windows specific:

Category: compiler
Name: tests/compiler/tcmdline_import_std_prefix.nim c -d:nimPreviewSlimSystem --warning:StdPrefix:on --warningAsError:StdPrefix:on --import:std/objectdollar
Action: run
Result: reNimcCrash
-------- Expected -------

--------- Given --------
$ D:\a\1\s\bin\nim.exe c --hints:on -d:testing --nimblePath:build/deps/pkgs2 --nimCache:nimcache\tests\compiler\tcmdline_import_std_prefix.nim_4a8a08f09d37b73795649038408b5f33 -d:nimPreviewSlimSystem --warning:StdPrefix:on --warningAsError:StdPrefix:on --import:std/objectdollar tests\compiler\tcmdline_import_std_prefix.nim
Hint: used config file 'D:\a\1\s\config\nim.cfg' [Conf]
Hint: used config file 'D:\a\1\s\config\config.nims' [Conf]
Hint: used config file 'D:\a\1\s\tests\config.nims' [Conf]
Hint: used config file 'D:\a\1\s\tests\compiler\nim.cfg' [Conf]
tcmdline_import_std_prefix.nim(1, 2) Error: objectdollar needs the 'std' prefix [StdPrefix]

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