go/build: avoid setting Default.GOROOT when runtime.GOROOT() is empty
Previously, we called path.Clean on the value of runtime.GOROOT() even
if it was empty, which would set it explicitly to ".".
That would cause (*Context).importGo to assume that errors resolving
paths in GOROOT are fatal and return early:
https://cs.opensource.google/go/go/+/master:src/go/build/build.go;l=1121-1127;drc=
38174b3a3514629b84dcd76878b2f536b189dd7b
If we instead leave it empty (and are in module mode), then importGo
will fall back to letting the 'go' command resolve the path, which may
succeed if the 'go' command can infer the correct GOROOT (from its own
stamped-in default GOROOT or executable path).
Fixes #51483
Change-Id: I44dce7cec6c3d1c86670e629ddfbca8be461130c
Reviewed-on: https://go-review.googlesource.com/c/go/+/391805
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>