Fix #767: drop duplicates when merging '..._append' lists

[phverg]

Problem

Fixes #767.

A natural inheritance pattern using YAML merge plus ..._append:

sim_base: &sim_base
  filesets_append: [files_dv]
  ...
unit_test:
  <<: *sim_base
  filesets_append: [files_dv]

…leaves unit_test with filesets_append: [files_dv, files_dv] after
YAML's list merge. Every file in files_dv then ends up listed twice in
the emitted EDAM, and the EDA backend rejects the build with a
file already defined error. The same happens for files_append
re-listing a file already present in files. The natural inheritance
pattern is effectively unusable.

Solution

Deduplicate when extending the base list inside
CoreData._append_lists. Items that are already present (in the base
list or earlier in the same ..._append) are dropped; new items are
still appended in order. This keeps the existing append-only and
base-only test cases unchanged.

Test plan

• Extended tests/capi2_cores/misc/append.core with three new
  targets covering the duplicate cases (in-base, within-append,
  append-only).
• Extended test_capi2_append to assert duplicates are dropped and
  order preserved; existing assertions still pass.
• Manually verified the issue scenario (YAML merge + double
  filesets_append): EDAM file list now contains each file once.
• Full pytest suite green locally.
• pre-commit run -a clean.

[olofk]

I understand what this fixes, but thinking about it I can't see how on earth we would get into this situation. The provided example kind of undermines the whole idea of inheritance as the point is that you would not have to add the fileset in the target that inherits. I realize your example is intentionally contrived, but could you (or someone else) come up with a more realistic situation where this could happen?