cmd/go: add source file contents to plugin hash

It is common to have multiple plugins built from ephemeral
source files all with the same name:

	# generate main.go
	go build -buildmode=plugin -o=p1.so main.go
	# rm main.go, generate new main.go
	go build -buildmode=plugin -o=p2.so main.go
	...

These different plugins currently have the same build ID,
and hence the same package path. This means only one can be
loaded.

To remove this restriction, this commit adds the contents of the
main package source files to the plugin hash.

Fixes #19358

Change-Id: Icd42024b085feb29c09c2771aaecb85f8b528dd3
Reviewed-on: https://go-review.googlesource.com/61170


Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>