Fix compilation with backtraces disabled part 2

[msimberg]

#1178 was incomplete. I've rearranged headers a bit so that pika/debugging/backtrace/backtrace.hpp is never included directly. It's not meant for end-users, but it's also not meant for internal use, except for inclusion in pika/debugging/backtrace.hpp. I've move the header to pika/debugging/detail/backtrace.hpp instead. This fixes compilation of the header tests.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.10% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7896e25)	18541	13853	74.72%
Head commit (f6fd282)	18541 (+0)	13871 (+18)	74.81% (+0.10%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1196)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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