Rewrite the logic to resolve package-relative paths so we can remove the "asset" library

[klamann]

Currently we use the asset library in exactly one place to figure out the path to a file inside an installed module. The asset library is GPL licensed and brings with it a few more GPL dependencies. As pyhocon is Apache licensed, this causes issues for pyhocon users that cannot include software with strong copyleft licenses like the GPL.

Also, it introduces a lot of transitive dependencies:

pyhocon==0.3.56
  - asset [required: Any, installed: 0.6.13]
    - aadict [required: >=0.2.2, installed: 0.2.3]
      - six [required: >=1.6.0, installed: 1.15.0]
    - globre [required: >=0.1.5, installed: 0.1.5]
    - six [required: >=1.10.0, installed: 1.15.0]
  - pyparsing [required: >=2.0.3, installed: 2.4.7]

Without asset, this would be reduced to

pyhocon==0.3.56
  - pyparsing [required: >=2.0.3, installed: 2.4.7]

In this PR I re-implemented the functionality of the asset.load() function so we can avoid using the asset library in the first place.

The implementation is really basic, because it relies on imp.find_module from the standard library. I would have preferred to use importlib.util.find_spec, but it's not available in Python 2.7. I also added a bunch of tests (the previous test was pretty much non-functional because it mocked the result of asset.load).

Solves #234

[coveralls]

Coverage increased (+0.03%) to 95.354% when pulling aaf8ba8 on klamann:feature/include-package-without-asset into c2fba83 on chimpler:master.

[darthbear]

Awesome work @klamann! Thanks for doing it!