lang/rust, lang/rust-nightly: Generate PLIST in post-install

[dumbbell]

Summary

Several libraries hae their filename computed and based on the absolute path to source files. Therefore, we need to generate the PLIST. Rust installer already produces manifests listing files it installs. The port now uses those files to complete ${TMPPLIST}.

Handling of DOCS-specific files is also unified with normal files. This gets rid of code duplication.

Test Plan

Tested locally and in Poudriere where paths are different in both cases

[dumbbell]

Forgot to mention: this fixes a staging failure for many people building the port outside of Poudriere, because the filenames depend on the source files location. Here is the Bugzilla report related to this:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217309

The patch was tested successfully by one of the reporters.

[riggs]

lang/rust/Makefile

211

Sorry for being picky, but these seem like pure whitespace changes. Can you revert those changes?

[dumbbell]

Revert a whitespace change

[antoine]

Does upstream plan to provide a way to have reproducible builds?

[dumbbell]

I don't know, I will ask them about that as I have the same concern.