File paths in --target are handled in a sub-par manner

[Tobba]

Currently if a file path is passed to --target it will be used as the target name. This has a few nasty side effects:

• If relative paths are used different relative paths pointing to the same file will be seen as different targets
• Relative paths pointing to different files may result in a match
• If absolute paths are used the target specifications may not be moved around the filesystem without doing a full rebuild
• The path to the target specification is leaked all over the place, and target-related errors become verbose

[Tobba]

The workaround for this is using the (mostly) undocumented RUST_TARGET_PATH environment variable.

[steveklabnik]

Triage: no change

[Mark-Simulacrum]

I'm going to close in favor of #24666.