Allow over-writing existing targets

[kunalb]

Description

Depending on the conda environment, the same file may be written to multiple times (eg. if a specific lib.so file exists, multiple conda packages may have it and at pack time conda-pack will pull from the cache'd original copy of the package and copy it in; when multiple packages have the same lib.so file they'll over-write each other).

shutil.copy2 doesn't handle overwriting existing symlinks cleanly: it overwrites files silently but fails on creating a symlink (probably worth a separate PR to cpython later). Allow for this by explicitly deleting existing targets.

Checklist - did you ...

• [x] Add a file to the news directory (using the template) for the next release's release notes?
  • Doesn't seem important enough
• [ ] Add / update necessary tests?
  • I'll just check what exists and update
• [x] Add / update outdated documentation?

[conda-bot]

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