adds src entry path and infers from shard.yml

[samueleaton]

notice in the following example how the entry path has /server/ in the directory structure. Before, Sentry would fail because it just defaulted to src/my_app.cr.

This change will be more flexible and will automatically pull in the src path from the targets object in the shard.yml file.

name: my_app
version: 0.1.0

authors:
  - Sam Eaton <sam@eaton.party>

targets:
  my_app:
    main: src/server/my_app.cr

crystal: 1.2.2