[config:export] fix path construction (#4245)

[cburschka]

• Use Drupal root, not Composer root.
• Avoid adding the root to an already absolute path.
• Avoid adding the root twice when creating the directory

Fixes #4245, fixes #4195, fixes #4198, fixes #4219

[cburschka]

Note: The third of the fixes in this PR is also contained in #4219, and should be credited to @timotheemoulin.

[timotheemoulin]

I'm glad someone else took the lead on this issue