x/tools/gopls: packages.Load calls for go.mod file

[stamblerre]

I just saw this in a log after opening a go.mod file.

[Info  - 11:08:05 PM] 2020/10/13 23:08:05 go/packages.Load
	snapshot=283
	directory=/var/folders/c5/cdxx0hzn6v3cvfn42kwt2tj000crcx/T/gopls-mod182702246
	query=[file=/Users/rstambler/code/src/github.com/julieqiu/opensourcelive/go.mod]
	packages=0

[gopherbot]

Change https://golang.org/cl/263983 mentions this issue: internal/lsp: do not allow non-Go files to be loaded in packages.load