ignore unknown srcs when querying whatinputs in package driver

[goddenrich]

previously if there was a go source file which wasn't included in a BUILD file target then the package driver would error with the following error:

error: ./driver-0.7.0-linux_amd64: exit status 1: 16:08:02.056 CRITICAL: Failed to load packages: exit status 1 Stdout:
16:08:02.016 CRITICAL: vault/core/adjustments/e2e/premerge_test.go is not a source to any current target

The package driver should not care about source files which haven't been added to targets.