package -f blob/blob.tgz didn't work as expected

[drnic]

$ tree blobs
blobs
└── softhsm
    └── SoftHSMv2-2.3.0.tar.gz

$ bosh-gen package softhsm -f softhsm/SoftHSMv2-2.3.0.tar.gz
      create  packages/softhsm/packaging
tar: Error opening archive: Failed to open 'softhsm/SoftHSMv2-2.3.0.tar.gz'
Skipping unknown file /Users/drnic/Projects/bosh_releases/softhsm-boshrelease/softhsm/SoftHSMv2-2.3.0.tar.gz
      create  packages/softhsm/spec

The way that worked was to include blobs/ - but it would be good to support existing blobs

$ bosh-gen package softhsm -f blobs/softhsm/SoftHSMv2-2.3.0.tar.gz