cmd/go/internal/modindex: don't write index entry if file open
On Windows, we can't open a file that's already been opened. Before this
change, we'd try to write an index entry if mmapping the entry failed.
But that could happen either if the file doesn't exist or if there was a
problem mmapping an already opened file. Pass through information about
whether the file was actually opened so that we don't try to write to an
already opened file.
For #71059
Change-Id: I6adabe1093fed9ec37e7fafb13384c102786cbce
Reviewed-on: https://go-review.googlesource.com/c/go/+/640577 Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>