cmd/go2nix: preserve quoted ldflags

[ryantm]

Parse ldflags with shell-style quoting before invoking the linker so values like -extldflags '-static -L…' stay intact.

[aldoborrero]

Thanks for tackling this — quoted -extldflags was a real gap.

One blocking issue: the new splitShellFields treats \ as an escape character (both unquoted and inside "..."), which diverges from go build -ldflags and is a regression vs main:

input	main (strings.Fields) / go build	this PR
-X main.Path=C:\Users\me	main.Path=C:\Users\me	main.Path=C:Usersme
-extldflags "-L C:\lib"	-L C:\lib	-L C:lib

go build -ldflags parses with cmd/internal/quoted.Split, whose doc comment is explicit: "There is no unescaping or other processing within quoted fields." (quoted.go:21-22). It only handles '/" delimiters and \ \t\n\r separators — no backslash escaping at all. Wired to -ldflags at load/flag.go:71.

There's a second, smaller divergence: quoted.Split only treats a quote as special at the start of a field (mid-token quotes are literal), whereas splitShellFields toggles quoting anywhere. So a"b c"d parses differently too.

Suggestion: port cmd/internal/quoted.Split directly (~35 lines, BSD-3; Go itself copies it verbatim in cmd/dist/quoted.go). That gives exact go build parity, fixes the backslash regression, and is simpler than the sh-style parser. The motivating -extldflags '-static -L/foo/lib' case works fine under quoted.Split since the quote is at field start.

Worth adding test cases for: literal backslash (would have caught the above), mid-token quote, adjacent quotes, and the unterminated-escape error path.

[aldoborrero]

This (and the case '\\': escaped = true at L376 inside "...") is the regression: go build -ldflags uses cmd/internal/quoted.Split, which has no escape handling — \ is always literal. Any ldflags containing a literal backslash (Windows path in -X, etc.) now gets it silently stripped, where both main and go build preserved it.

[aldoborrero]

Consider replacing this with a port of cmd/internal/quoted.Split (with attribution) so we match go build -ldflags exactly rather than POSIX-sh semantics — that's the contract buildGoModule users are used to. It also avoids the backslash issue and the mid-token-quote divergence.

[aldoborrero]

Would be good to add a case with a literal backslash, e.g. {flags: []string{"-X main.Path=C:\\Users"}, want: []string{"-X", "main.Path=C:\\Users"}} — that's the case that regresses with the current escape handling.

[aldoborrero]

Thanks again for this — picked it up in #66 since the branch couldn't be pushed to. Your commit is cherry-picked as-is (authorship preserved); the follow-up commit swaps the splitter for a port of cmd/internal/quoted.Split per the review above. Closing in favour of #66.