fix(cmd): CLI Config Relative Imports

[jshlbrd]

Description

Closes #275 by supporting relative imports.

Motivation and Context

This fixes the relative import problem by switching the directory to each config (jsonnet, libsonnet) file's directory during every test or vet call.

How Has This Been Tested?

Tested with an internal repo that has 10s to 100s of config files containing relative imports (substation test -R, substation vet -R).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[jtan-brex]

Tested and work as expected! left two comments about adding default case in switch filepath.Ext(path) for both test and vet CLI