restructure all service-specific code out of main functions

[mpadge]

... and into sub-functions containing in distinctly-named files. All in preparation for #41

[mpadge]

All service-specific functionality is now contained in the files named "service-*.R":

flist <- list.files ("./R", pattern = "^service")
print (flist, width = 20)
#> [1] "service-desc-meta.R"
#> [2] "service-functions.R"
#> [3] "service-methods.R"  
#> [4] "service-urls.R"

s <- pkgstats::pkgstats ()
objs <- s$objects [which (s$objects$file_name %in% paste0 ("R/", flist)), ]
serv_pc <- round (100 * sum (objs$loc) / sum (s$objects$loc), digits = 1L)
message ("service-specific code is ", serv_pc, "%")
#> service-specific code is 8.3%

^{Created on 2023-06-14 with reprex v2.0.2}

That is < 10% of all code, so should now be pretty easy to integrate additional services.

---

EDIT: Additional service-specific functions are also in:

• "httptest-mocks.R"
• "client-private-dcmi2host.R"
• "client-private-embargo-publish.R"
• "upload-<zenodo/figshare>.R"
• "search-params.R"

That makes 9 files in total which have to be modified. But those latter five are all for specific methods, and should generally be fairly straightforward. I think this can be left for now, and maybe modified, improved, or streamlined while addressing #41.