Removing Sphinx dependency from main package

[brimoor]

Resolves #513.

Refactors heavier dependencies like Sphinx that are only required when using ETA's pipeline features into a new [pipeline] extras package.

Also adds some additional features to install_package(), ensure_package(), and ensure_import() that are used by voxel51/fiftyone#1059.

git clone https://github.com/voxel51/eta
cd eta

# lite installation
pip install -e .

import eta.core.utils as etau

etau.ensure_package(["numpy", "scipy"])  # success
etau.ensure_import(["numpy", "scipy"])  # success

etau.ensure_package(["numpyyy", "scipy>1000"])  # helpful error
etau.ensure_import(["numpyyy", "scipy>1000"])  # helpful error

# do something that requires [pipeline] extras
eta modules --metadata eta/modules/sample_videos.py  # helpful error

pip install -e .[pipeline]

eta modules --metadata eta/modules/sample_videos.py  # works

[benjaminpkane]

LGTM