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

Several libraries have 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}.

While the port built fine in Poudriere, it failed when built directly on
the host (regular make or with portmaster(1)) or using a different tool
such as Synth. This commit fixes the build for those methods.

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

How to do reproducible builds will be studied later.

PR: 217309
Reported by: Several people on freebsd-ports@ or Bugzilla
Tested by: Almost everyone who reported the issue
Approved by: antoine (mentor), riggs (maintainer of lang/rust)
Differential Revision: https://reviews.freebsd.org/D9816