chore: Avoid submodules for building igraph

[krlmlr]

We keep using the submodules, but only as a source of the C files. The process of updating the C core is documented in src/README.md .

[codecov]

Codecov Report

Merging #674 (678d796) into main (438c21d) will not change coverage.
The diff coverage is n/a.

❗ Current head 678d796 differs from pull request most recent head 6d90984. Consider uploading reports for the commit 6d90984 to get more accurate results

@@           Coverage Diff           @@
##             main     #674   +/-   ##
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  Coverage   60.48%   60.48%           
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  Files          73       73           
  Lines       21608    21608           
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  Hits        13070    13070           
  Misses       8538     8538           

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[szhorvat]

Makes sense to me, though @ntamas is much more qualified to comment on this type of thing :-)

[szhorvat]

Was this line meant to be left in?

[krlmlr]

We could replace it by an example to update to a specific SHA. I'm not familiar with git submodule syntax.

[szhorvat]

Ah, that's what you meant. I'll complement this README file later to make that clear, but feel free to merge anytime.