cmd/go: fix 'go test' and 'go fmt' with files outside a module

Use the actual loader result in findModule instead of making
assumptions about nesting in the build list.
As a side-effect, this produces much clearer error messages for
packages that (for one reason or another) failed to load.

Adjust the package and module path outside a module to
"command-line-arguments". That string already appears in the output of
a number of (module-mode and GOPATH-mode) commands for file arguments,
and as far as I can tell operation outside a module is currently the
only case in which the module path of a package is not actually a
prefix of the import path.

Fixes #28011
Fixes #27099
Fixes #28943
Updates #27102
Updates #28459
Updates #27063

Change-Id: I61d5556df7b1b7d1efdaffa892f0e3e95b612d87
Reviewed-on: https://go-review.googlesource.com/c/153459
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/cmd/go/testdata/script/mod_outside.txt b/src/cmd/go/testdata/script/mod_outside.txt
index 25013b6271..db994a1656 100644
--- a/src/cmd/go/testdata/script/mod_outside.txt
+++ b/src/cmd/go/testdata/script/mod_outside.txt
@@ -12,8 +12,8 @@ stdout 'NUL|/dev/null'
 # which is not in a module.
 ! go list
 stderr 'cannot find main module'
-! go list -m
-stderr 'cannot find main module'
+go list -m
+stdout '^command-line-arguments$'
 # 'go list' in the working directory should fail even if there is a a 'package
 # main' present: without a main module, we do not know its package path.
 ! go list ./foo
@@ -148,6 +148,10 @@ stderr 'no such package'
 stderr 'can only use path@version syntax with'
 
 
+# 'go fmt' should be able to format files outside of a module.
+go fmt foo/foo.go
+
+
 # The remainder of the test checks dependencies by linking and running binaries.
 [short] stop
 
@@ -185,8 +189,8 @@ stdout 'using example.com/version v1.0.0'
 
 # 'go run' should use 'main' as the effective module and import path.
 go run ./foo/foo.go
-stdout 'path is \.$'
-stdout 'main is main \(devel\)'
+stdout 'path is command-line-arguments$'
+stdout 'main is command-line-arguments \(devel\)'
 stdout 'using example.com/version v1.1.0'
 
 # 'go generate' should work with file arguments.