Fix import command so it works if Lerna root is a subdir of git root

[RoystonS]

Fix for #1197

Description

The code in the import command was computing a targetDir relative to the Lerna root directory, and using that in the Git patches. If the Lerna root directory wasn't in the Git root directory, that produced incorrect import locations.

Motivation and Context

This fix is needed in order to enable Lerna to import correctly if Lerna is not based in the root of the Git workspace.

Fixes issue #1197

How Has This Been Tested?

1. New unit test added to import command tests. The test creates a Lerna environment where Lerna is in a subdirectory of the Git workspace, and checks that the import works correctly. It failed before I fixed the code, and now passes. I've added a sibling 'describe' that configures an environment where Lerna isn't in the root, in order to avoid touching all the existing tests.
2. I've manually run the resulting Lerna build on a real-world workspace example I had that previously produced the wrong output. It's now correct.

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.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[evocateur]

LGTM

[lock]

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