community/gopls: upgrade to 0.16.1

https://github.com/golang/tools/releases/tag/gopls%2Fv0.16.1

We have to add the !check option because the test directory (see cd test in check function) was removed in v0.15.0: https://github.com/golang/tools/commit/00ec6daca3ac

I tried using go test "$builddir"/... in the check function, but that yielded many test failures for very unclear reasons. For example:

--- FAIL: TestFreeRefs (0.00s)
    --- FAIL: TestFreeRefs/file2.go (0.00s)
        freesymbols_test.go:107: file2.go:1:19: could not import fmt (can't find import: "fmt": cannot find package "fmt" in any of:
        	    ($GOROOT not set)
        	    /home/foo/go/src/fmt (from $GOPATH))
        freesymbols_test.go:107: file2.go:1:43: x declared and not used
        freesymbols_test.go:128: (-want +got)
              []string{
            - 	"file pkgname fmt.Stringer",
            + 	"file pkgname fmt",
              }
    --- FAIL: TestFreeRefs/file4.go (0.00s)
        freesymbols_test.go:107: file4.go:1:19: could not import bytes (can't find import: "bytes": cannot find package "bytes" in any of:
        	    ($GOROOT not set)
        	    /home/foo/go/src/bytes (from $GOPATH))
        freesymbols_test.go:128: (-want +got)
              []string{
            - 	"local var buf.WriteByte",
            + 	"local var buf",
            - 	"local var buf.WriteString",
            + 	"local var buf",
              }
    --- FAIL: TestFreeRefs/file5.go (0.00s)
        freesymbols_test.go:107: file5.go:1:21: could not import errors (can't find import: "errors": cannot find package "errors" in any of:
        	    ($GOROOT not set)
        	    /home/foo/go/src/errors (from $GOPATH))
        freesymbols_test.go:107: file5.go:1:41: undefined: New
        freesymbols_test.go:128: (-want +got)
              []string(
            - 	{"file pkgname errors.New"},
            + 	nil,
              )
    --- FAIL: TestFreeRefs/file6.go (0.00s)
        freesymbols_test.go:107: file6.go:1:19: could not import net/url (can't find import: "net/url": cannot find package "net/url" in any of:
        	    ($GOROOT not set)
        	    /home/foo/go/src/net/url (from $GOPATH))
        freesymbols_test.go:128: (-want +got)
              []string{
            - 	"file pkgname url.URL",
            + 	"file pkgname url",
              }
    --- FAIL: TestFreeRefs/file7.go (0.00s)
        freesymbols_test.go:107: file7.go:1:21: could not import net/url (can't find import: "net/url": cannot find package "net/url" in any of:
        	    ($GOROOT not set)
        	    /home/foo/go/src/net/url (from $GOPATH))
        freesymbols_test.go:107: file7.go:1:41: undefined: URL
        freesymbols_test.go:128: (-want +got)
              []string(
            - 	{"file pkgname url.URL"},
            + 	nil,
              )
    --- FAIL: TestFreeRefs/file9.go (0.00s)
        freesymbols_test.go:107: file9.go:1:19: could not import go/build (can't find import: "go/build": cannot find package "go/build" in any of:
        	    ($GOROOT not set)
        	    /home/foo/go/src/go/build (from $GOPATH))
        freesymbols_test.go:128: (-want +got)
              []string{
            - 	"file pkgname build.Default.GOOS",
            + 	"file pkgname build",
              }
FAIL
FAIL	golang.org/x/tools/gopls/internal/golang	0.045s

The fmt, byte, errors, etc. packages are all a part of the golang standard library, so I'm not sure what the problem is.