x/tools/go/packages/packagestest: build mostly broken on Windows

[bradfitz]

The x/tools trybots are usually(?) failing on Windows with:

ok  	golang.org/x/tools/go/loader	13.435s
--- FAIL: TestReturnErrorWhenUsingNonGoFiles (0.09s)
    --- FAIL: TestReturnErrorWhenUsingNonGoFiles/Modules (0.05s)
        packages_test.go:1799: go [mod download]: exit status 1: go: golang.org/gopathb@v1.0.0: proxy URL "file://C:/Users/gopher/AppData/Local/Temp/1/TestReturnErrorWhenUsingNonGoFiles_Modules105513718/modproxy" uses file scheme with non-path elements
            
FAIL
FAIL	golang.org/x/tools/go/packages	41.030s
?   	golang.org/x/tools/go/packages/gopackages	[no test files]
--- FAIL: TestModulesExport (0.17s)
    modules_test.go:17: go [mod download]: exit status 1: go: golang.org/fake2@v1.0.0: proxy URL "file://C:/Users/gopher/AppData/Local/Temp/1/TestModulesExport123494941/modproxy" uses file scheme with non-path elements
        
FAIL
FAIL	golang.org/x/tools/go/packages/packagestest	0.448s

It's almost entirely red for Windows at:

https://build.golang.org/?repo=golang.org%2fx%2ftools

This is causing development problems for people using trybots at:

https://go-review.googlesource.com/c/tools/+/1642

[gopherbot]

Change https://golang.org/cl/173499 mentions this issue: cmd/go/internal/modfetch: less restrictive file URL parsing

[gopherbot]

Change https://golang.org/cl/173918 mentions this issue: x/tools/go/packages/packagestest: fix GOPROXY file URLs for Windows

[jayconrod]

This should be fixed by the two CLs above.

[gopherbot]

Change https://golang.org/cl/173919 mentions this issue: go/packages/packagestest: fix GOPROXY file URLs for Windows