fix combination of --copy-ec and --from-pr

[boegel]

This continues from the work done by @ocaisa in #3473, and fixes the combination of --copy-ec and --from-pr.

The scope of this PR is pretty big, and I feel it should be split up into several PRs, so I made this a draft PR for now.

High level overview of additional changes:

• enhance --copy-files function so it can be used to implement --copy-ec (both with and without --from-pr)
  • => fleshed out to PR enhance copy_files: single file target, error on empty input list, verbose mode #3483
• implement caching of result of fetch_files_from_pr calls (mostly to significantly speed up tests)
  • => fleshed out to PR cache result of fetch_files_from_pr function #3484
• add locate_files function to easybuild.tools.filetools module which can be leverared in det_easyconfig_paths function
  • => fleshed out to PR add locate_files function to filetools module #3485
• avoid that download_repo function from easybuild.tools.github module changes working directory
  • => fleshed out to PR don't change directory in download_repo function in tools.github #3486
• add det_copy_ec_specs function to leverage from main.py
• clean exit after handling combination of --copy-ec and --try-*
• fix default for pr_target_account build option (only really relevant for tests)
• enhance tests to cover all these changes & fixes

fixes #3178

[boegel]

@ocaisa This should be good to go now, can you give it another look?

[ocaisa]

Looks good, just a typo, will take a deeper look while you fix that

[ocaisa]

Looks good to me, one minor request

[migueldiascosta]

also lgtm, fwiw (and more importantly, it works! :) )