feat(services/building/configuration): load targets dotenv files

[homer0]

What does this PR do?

This is the implementation of homer0/projext#80.

It add supports for targets' .env files.

When creating the definitions that end up on webpack.DefinePlugin, WebpackConfiguration will now include the variables from the .env files.

How should it be tested manually?

In order to test this, you need to install homer0/projext#next and make sure you delete the copy of projext npm/yarn will install inside this package's node_modules.

Create an .env.[target-name] file with a variable declaration and then log it on your target using process.env.[VARIABLE-NAME].

And, of course...

yarn test
# or
npm test

[coveralls]

Pull Request Test Coverage Report for Build 263

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 260:	0.0%
Covered Lines:	609
Relevant Lines:	609

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💛 - Coveralls