internal/localdatasource: correctly initialize DataSource.sourceClient
internal/localdatasource: correctly initialize DataSource.sourceClient

Create a valid source.Client to avoid nil pointer dereference while
processing packages.

Change-Id: Id19a2ba07cfac8de85c9386a069d100b907421d2
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/280619
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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