internal/frontend: do not set SearchMode by default
internal/frontend: do not set SearchMode by default

By default, set m="" instead of m="package".

For golang/go#44142

Change-Id: I2a9c53a4a5bf7f9cd7056996621ef82b0e0f25d2
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/346534
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite