"dbt seed" with dependencies -> error in path resolution

[beckjake]

Describe the bug

I have a project with a dependency that has seeds. When I run dbt seed in the parent project, I get an error about the dependency seeds:
[Errno 2] No such file or directory: 'data/seedtable.csv'

Steps To Reproduce

1. Create a project with data/seedtable.csv named a
2. Create another project named b and with a packages.yml like:

packages:
  - local: /path/to/project/a

3. run dbt deps && dbt seed from the project b directory
4. feel sad that it gives you an error

Expected behavior

I expect my dependency's seed to run successfully.

Screenshots and log output

2019-09-04 16:36:25,854704 (MainThread): Acquiring new postgres connection "seedtable".
2019-09-04 16:36:25,854894 (MainThread): Re-using an available connection from the pool (formerly model_2).
2019-09-04 16:36:25,963811 (MainThread): Unhandled error while executing seed.minimal.seedtable
[Errno 2] No such file or directory: 'data/seedtable.csv'
2019-09-04 16:36:25,964249 (MainThread):
Traceback (most recent call last):
  File "/Users/jake/src/fishtown/dbt/core/dbt/node_runners.py", line 203, in safe_run
    result = self.compile_and_execute(manifest, ctx)
  File "/Users/jake/src/fishtown/dbt/core/dbt/node_runners.py", line 146, in compile_and_execute
    result = self.run(ctx.node, manifest)
  File "/Users/jake/src/fishtown/dbt/core/dbt/node_runners.py", line 245, in run
    return self.execute(compiled_node, manifest)
  File "/Users/jake/src/fishtown/dbt/core/dbt/node_runners.py", line 357, in execute
    materialization_macro.generator(context)()
  File "/Users/jake/src/fishtown/dbt/core/dbt/clients/jinja.py", line 127, in call
    return macro(*args, **kwargs)
  File "/Users/jake/.pyenv/versions/3.6.5/envs/dbt36/lib/python3.6/site-packages/jinja2/runtime.py", line 575, in __call__
    return self._invoke(arguments, autoescape)
  File "/Users/jake/.pyenv/versions/3.6.5/envs/dbt36/lib/python3.6/site-packages/jinja2/asyncsupport.py", line 110, in _invoke
    return original_invoke(self, arguments, autoescape)
  File "/Users/jake/.pyenv/versions/3.6.5/envs/dbt36/lib/python3.6/site-packages/jinja2/runtime.py", line 579, in _invoke
    rv = self._func(*arguments)
  File "<template>", line 346, in macro
  File "/Users/jake/.pyenv/versions/3.6.5/envs/dbt36/lib/python3.6/site-packages/jinja2/sandbox.py", line 438, in call
    return __context.call(__obj, *args, **kwargs)
  File "/Users/jake/.pyenv/versions/3.6.5/envs/dbt36/lib/python3.6/site-packages/jinja2/runtime.py", line 262, in call
    return __obj(*args, **kwargs)
  File "/Users/jake/src/fishtown/dbt/core/dbt/context/common.py", line 364, in load_agate_table
    table = dbt.clients.agate_helper.from_csv(path)
  File "/Users/jake/src/fishtown/dbt/core/dbt/clients/agate_helper.py", line 65, in from_csv
    with open(abspath, encoding='utf-8') as fp:
FileNotFoundError: [Errno 2] No such file or directory: 'data/seedtable.csv'

System information

Which database are you using dbt with?
Any

The output of dbt --version:
0.15.0-a1

The operating system you're using:
Any

The output of python --version:
Any

Additional context

I'm pretty sure this is related to the new FilePath construct I introduced. It doesn't know about the project root of the project it was parsed from, so original_file_path (which we use when loading seeds at runtime) is a relative path. I think the proper fix is for FilePath to know about the project_root of the project it came from and construct the original_file_path property in a way that includes the project_root.

This is only a problem for seeds, as all other files are read in during parse time.