Improve build times through header removal and relocation

- Eliminate unused headers
- reduce dependencies among headers (slightly)

The include-what-you-use tool (IWYU) was used to analyze header
dependencies and recommend fixes; some additional manual work was
then performed.

Add missing include
2 jobs for header-cleanup in 156 minutes and 13 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed #4824
amd64
job_clang_amd64

02:36:12

passed #4823
amd64
job_gcc_amd64

02:15:51