Support recreation of wrapper scripts in deployed environments

[ncoghlan]

Most of the scripts deleted for archive portability and reproducibility reasons could be recreated in postinstall.py from the original entry points distribution metadata.

Restoring these could be enabled via an opt-in flag in the layer spec (omitting them would still be the default behaviour).

[ncoghlan]

Note that this feature proposal does NOT cover adding shebang line rewriting support for published scripts that aren't automatically generated based on distribution entry point metadata.

Adding that would require supporting shebang line rewriting and RECORD file editing in the deployed environment (by contrast, #28 only proposes supporting RECORD file editing at stack build time)