fix setCommand so it behaves like regular nim invocation and so that projectName, outFile, json file etc are correct

[timotheecour]

• fix issue 2 (see nimyaml doesn't work on devel anymore #18543 (comment))
• remove code duplication between processCmdLineAndProjectPath and setCommand
• paths set after setCommand are now correct and consistent with what you'd have if you'd use a regular nim compilation command, as can be seen with adding this to test/tlex.nim in https://github.com/flyx/NimYAML:

import std/compilesettings
static: echo "recompiling"
static: echo (querySetting(outFile), querySetting(outDir), querySetting(projectName), querySetting(projectPath), querySetting(projectFull), querySetting(command))

before PR, those paths were wrong (outFile is assumed to be base name in rest of code):
("test/tlex", "/private/tmp/d13/NimYAML", "test/tlex", "/private/tmp/d13/NimYAML", "/private/tmp/d13/NimYAML/test/tlex", "c")

after PR, those paths are correct:
("tlex", "/private/tmp/d13/NimYAML/test", "tlex", "/private/tmp/d13/NimYAML/test", "/private/tmp/d13/NimYAML/test/tlex", "c")

and they're the same as what you'd have with:
nim c -r --nimcache:/tmp/c09f test/tlex

note

before #18100, writeJsonBuildInstructions would silently not write the json file (because of the above bug in setCommand which resulted in a wrong value for jsonBuildInstructionsFile); after #18100 it would crash as mentioned in #18543 (comment) as a result of this;
after this PR, the paths computed after setCommand are now correct, so jsonBuildInstructionsFile is also correct and the json file is indeed written (and -d:nimBetterRun is honored)

azure CI keeps getting cancelled for some unrelated reason

##[error]An error occurred while provisioning resources (Error Type: Failure).
,##[error]The remote provider was unable to process the request.

EDIT: upstream issue, refs https://docs.microsoft.com/en-us/answers/questions/484649/an-error-occurred-while-provisioning-resources-err.html

[Araq]

Superb work, thanks!