Not possible to use absolute path to sources

**Actual Behaviour**

Failure when trying to use absolute path to sources (`/tmp/users` below).

**Expected Behaviour**

It should be possible to use absolute path to sources.

**Steps to reproduce it**

serverless.yaml:

```yaml
custom:
dynamodb:
stages:
- dev
start:
port: 8000
migrate: true
convertEmptyValues: true
noStart: false
inMemory: true
sharedDb: true
seed: true
seed:
test:
sources:
- table: users
sources: [/tmp/users]
```

**Log**

```
Dynamodb Local Started, Visit: http://localhost:8000/shell
Serverless: DynamoDB - created table users
Error --------------------------------------------------
Error: source file /home/user/project/tmp/users does not exist
at /home/user/project/node_modules/serverless-dynamodb-local/src/seeder.js:138:15
at tryCatcher (/home/user/project/node_modules/bluebird/js/release/util.js:16:23)
at Promise._settlePromiseFromHandler (/home/user/project/node_modules/bluebird/js/release/promise.js:547:31)
at Promise._settlePromise (/home/user/project/node_modules/bluebird/js/release/promise.js:604:18)
at Promise._settlePromise0 (/home/user/project/node_modules/bluebird/js/release/promise.js:649:10)
at Promise._settlePromises (/home/user/project/node_modules/bluebird/js/release/promise.js:729:18)
at _drainQueueStep (/home/user/project/node_modules/bluebird/js/release/async.js:93:12)
at _drainQueue (/home/user/project/node_modules/bluebird/js/release/async.js:86:9)
at Async._drainQueues (/home/user/project/node_modules/bluebird/js/release/async.js:102:5)
at Immediate.Async.drainQueues [as _onImmediate] (/home/user/project/node_modules/bluebird/js/release/async.js:15:14)
at processImmediate (internal/timers.js:439:21)
at process.topLevelDomainCallback (domain.js:131:23)
For debugging logs, run again after setting the "SLS_DEBUG=*" environment variable.
Get Support --------------------------------------------
Docs: docs.serverless.com
Bugs: github.com/serverless/serverless/issues
Issues: forum.serverless.com
Your Environment Information ---------------------------
Operating System: linux
Node Version: 12.13.0
Framework Version: 1.61.1
Plugin Version: 3.4.1
SDK Version: 2.3.0
Components Core Version: 1.1.2
Components CLI Version: 1.4.0
```

Start in seeder.js at the reported failure around line 138 and reproduce the issue with the serverless.yaml configuration shown. Check how the source path is resolved before the existence check. Done means an absolute source such as /tmp/users is used as written while relative paths continue to work.

Written by the indexing model from the issue text.