What exactly did you do, I can't reproduce the issue as described

Also note that names without a dot in the first element are considered reserved for the standard library, using a proper name gives a better error message

» mkdir /tmp/example && cd /tmp/example

» go mod init example.com/foo
go: creating new go.mod: module example.com/foo

» cat << EOF > main.go
package main
import _ "aaa"
func main() {}
EOF

» go build
main.go:3:8: package aaa is not in GOROOT (/home/arccy/sdk/gotip/src/aaa)

» mkdir vendor

» go build
main.go:3:8: cannot find package "." in:
	/tmp/example/vendor/aaa

You exactly reproduced the error described above.

I don't think it's a problem? Without vendor/ a package with a name like aaa cannot be retrieved by standard tooling so it makes sense to report it as not found in GOROOT. With vendor/, which can be created by other tools, it is possible to use a package named like that

OK, but shouldn't the message be

cannot find package "aaa" in:
	ProjectRoot/vendor

package "." confuses people.

that's #38748

Looks so. Close it for that.