Fix the path to the on-disk styles file for in-repo engines

[charlespierce]

Info

• When using in-repo engines, the path generated to the engine's styles is malformed, resulting in an error like:

Can't resolve '../../../account-engine/account-engine/__COMPILED_STYLES__/account-engine.css' in '$TMPDIR/embroider/8840c8/assets/_engine_'

• There appear to be two issues: First, by passing the filename into explicitRelative, the path has one too many ../ elements at the beginning (since it treats the file as a directory as well).
• Second, assuming that the engine will be in a top-level directory with the name equal to it's package name is wrong, as in-repo engines are contained in lib/{package-name}.

Changes

• Updated the implicit-styles code to call dirname on the underlying relativePath (stripping off the filename from the path)
• Updated the implicit-styles code to use engine.appRelativePath, instead of hard-coding it to be the package name for the engine, making sure it includes the lib/ if necessary.

Tested

• Ran a build with an in-repo engine to confirm that the styles can be located by Webpack.
• Updated the test scenarios to include an in-repo addon and confirm that it builds and works as expected.

[rwjblue]

Can you add an in-repo engine with styles to the engines-host-app?

[charlespierce]

Updated to include an in-repo addon in the engines-host-app project. Also manually confirmed that the test fails to build without the changes in this PR (for the reasons described).