Remove --root, require the cwd to be the root

This allows us to use relative paths everywhere.
It also produces more accurate emulation of TOPDIR,
which should be an empty string.

Bug: 213508006
Test: go test
Change-Id: Ie4e357687486e84e9f4aad0f6776d8feb2b9fc63

mk2rbc/mk2rbc_test.go

@@ -1387,7 +1387,6 @@ func TestGood(t *testing.T) {
 				ss, err := Convert(Request{
 					MkFile:         test.mkname,
 					Reader:         bytes.NewBufferString(test.in),
-					RootDir:        ".",
 					OutputSuffix:   ".star",
 					SourceFS:       fs,
 					MakefileFinder: &testMakefileFinder{fs: fs},