Pass absolute source paths to go tool cover

[aroulle-ubika]

go tool cover copies each source path it is given verbatim into a //line <path>:1:1 directive at the top of the file it generates.

Since go.dev/issue/70478, released in Go 1.27, the compiler resolves a relative filename in a //line directive against the directory of the file containing the directive rather than against the current directory. Absolute filenames are unaffected.

please_go passes relative paths, so from Go 1.27 onwards the package directory ends up duplicated in every position recorded for an instrumented file. Code calling runtime.Caller under plz cover sees paths like

lib/go/logger/lib/go/logger/logger_test.go:39

instead of

lib/go/logger/logger_test.go:39

which breaks tests asserting on caller information, and feeds wrong paths into coverage metadata. plz test is unaffected because it does not instrument.

Make the paths absolute before handing them over. While here, fix an adjacent bug: filepath.Join(dir, "_covervars.cover.go\n") ran the trailing newline through Clean and swallowed it, so the covervars entry and the first source shared a line in the output file list.

[aroulle-ubika]

@toastwaffle @samwestmoreland @Tatskaari

Could someone review, plz 👼

[toastwaffle]

@aroulle-ubika I'll review this on Tuesday. I'm a little concerned that using absolute paths like this will lead to problems with sandboxing and remote execution, but it is too late on a Friday for me to give this the required level of thought.