keg: mkpath while linking {include,lib,share}/postgresql@X

[cho-m]

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I think this will make it easier to retain extensions on different PostgreSQL.

citus, for example, installs files like:

• include/postgresql@14/server/citus_version.h
• lib/postgresql@14/citus.so
• share/postgresql@14/extension/citus--10.0-1--10.0-2.sql

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Reasoning here is we may need to manually link some keg_only files/dirs inside the post install of postgresql@X formulae; however, if another formula already created symlink paths then it will make things complicated.

I guess some other alternatives would be:

1. expose the keg.link_dir logic that does all the symlink-to-real-directory conversion. However, Keg is @api private and we may not want to change this.
   

   brew/Library/Homebrew/keg.rb

   Line 618 in c3094ac

   def link_dir(relative_dir, verbose: false, dry_run: false, overwrite: false)

2. extend keg_only DSL to allow "partial" keg_only. Con here is this will require various code modifications (across link code, DSL, JSON API, etc) for something that may only be useful for PostgreSQL.

[MikeMcQuaid]

Great idea and this makes sense to me, thanks @cho-m!