Fix output folder for absolute path inputs

[davispuh]

Currently if you use absolute path for input files then output folder is not respected.

That is, this fails:

$ dstep -o /tmp /usr/include/stdio.h /usr/include/stdlib.h 
dstep: an unknown error occurred: std.file.FileException@std/file.d(840): /usr/include/stdio.d: Permission denied
----------------
/usr/include/dlang/dmd/std/file.d:283 @trusted bool std.file.cenforce!(bool).cenforce(bool, scope const(char)[], scope const(char)*, immutable(char)[], ulong) [0x562fb05e0003]
??:? @trusted void std.file.writeImpl(scope const(char)[], scope const(char)*, scope const(void)[], bool) [0x562fb06dae0b]
/usr/include/dlang/dmd/std/file.d:745 @safe void std.file.write!(immutable(char)[]).write(immutable(char)[], const(void[])) [0x562fb0639917]
dstep/translator/Translator.d:75 void dstep.translator.Translator.Translator.translate() [0x562fb06b35e0]
dstep/driver/Application.d:66 void dstep.driver.Application.Application.run() [0x562fb065a15c]
dstep/driver/CommandLine.d:181 int dstep.driver.CommandLine.run(immutable(char)[][]) [0x562fb066aa61]
dstep/main.d:18 _Dmain [0x562fb066d977]
std.file.FileException@std/file.d(840): /usr/include/stdio.d: Permission denied
----------------
/usr/include/dlang/dmd/std/file.d:283 @trusted bool std.file.cenforce!(bool).cenforce(bool, scope const(char)[], scope const(char)*, immutable(char)[], ulong) [0x562fb05e0003]
??:? @trusted void std.file.writeImpl(scope const(char)[], scope const(char)*, scope const(void)[], bool) [0x562fb06dae0b]
/usr/include/dlang/dmd/std/file.d:745 @safe void std.file.write!(immutable(char)[]).write(immutable(char)[], const(void[])) [0x562fb0639917]
dstep/translator/Translator.d:75 void dstep.translator.Translator.Translator.translate() [0x562fb06b35e0]
dstep/driver/Application.d:66 void dstep.driver.Application.Application.run() [0x562fb065a15c]
dstep/driver/CommandLine.d:181 int dstep.driver.CommandLine.run(immutable(char)[][]) [0x562fb066aa61]
dstep/main.d:18 _Dmain [0x562fb066d977]

It tries to create .d files in same directory as input file without respecting output folder.
This PR fixes that so that it will correctly use output folder for such case.

[Copilot]

Pull request overview

Fixes output path generation so that using -o <dir> with absolute input header paths places generated .d files under the requested output directory instead of attempting to write next to the input headers (often unwritable system locations).

Changes:

• Add findBasePath helper to compute a common base directory for absolute inputs.
• Update multi-input output mapping to build outputs under config.output using relativePath(file, basePath).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
dstep/driver/Util.d	Adds findBasePath to derive a base directory from absolute input paths.
dstep/driver/Application.d	Uses findBasePath + relativePath when building output paths for multiple inputs.

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[jacob-carlborg]

There are no new tests added that shows the change is working.

[davispuh]

I rewrote this PR and added tests. Implement support for Windows aswell and tested that it works correctly.