Allow using files from the pool while importing source packages

Sometimes source packages reference files already present in the pool.

Allow for those file to be omitted when importing packages either via
`repo add` or `repo include`. If file is missing, aptly would make
an attempt to look up file in the package pool (by checksum) and
use it.

Fixes: #278

system/t09_repo/AddRepo16Test_gold

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+[+] pyspi_0.6.1-1.3_source added