cmd/{go,cover}: fix for -coverprofile path capture with local pkg
When coverage testing a local package (defined by a relative import
path such as "./foo/bar") the convention when "-coverprofile" is used
has been to capture source files by full pathname, as opposed to
recording the full import path or the invented import path
("command-line-arguments/") created by the go command in the case of
building named Go files. Doing this makes it much easier to use
collected profiles with "go tool -cover -html=<profile>".
The support for this feature/convention wound up being inadvertantly
dropped during the GOEXPERIMENT=coverageredesign implementation; this
patch restores it.
Fixes #56433.
Change-Id: Ib9556fdc86011b00c155caa614ab23e5148f3eb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/445917
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>