Changed the way bucket keys are generated from Source field.

The source field is now used to generate the bucket key, whereas previously
you had to modify Destination to change the bucket key prefix. The advantage of
this approach is you don't have to create a rule for each sub-directory.

Example:

Source: pool/main/k/**/*.deb
Destination: some-bucket

Local directory contents:
pool/main/k/kms-core-master/kms-core-master_1_amd64.deb
pool/main/k/kms-core-production/kms-core-production_1_amd64.deb

Results in:
s3://some-bucket/pool/main/k/kms-core-master/kms-core-master_1_amd64.deb
s3://some-bucket/pool/main/k/kms-core-production/kms-core-production_1_amd64.deb

Previously resulted in:
s3://some-bucket/kms-core-master_1_amd64.deb
s3://some-bucket/kms-core-production_1_amd64.deb