Revamp installed_on_request handling

[MikeMcQuaid]

• reinstall and upgrade no longer mark as installed on request, with or without names specified, but preserve the version from the tab instead
• default install_on_request to false rather than true
• only set installed in request in a tab if it's missing rather than false

@cho-m feel free to push any changes/test fixes directly to this PR as desired

Fixes #18754

[cho-m]

Okay with changes avoiding excess installed-on-request (given we often recommend brew [reinstall | upgrade] ... as a way of fixing some issues)

I added comments on behavior differences that may need to be verified as intentional.

[cho-m]

There is a scenario of brew reinstall <not-installed-formula>. Previously, this would behave like brew install. With change, the installed formula will be neither installed on request nor a dependency, so will be deleted on next brew autoremove

[cho-m]

Previous logic may be preferred so that brew install <already-installed-formula> has same tab as brew install <not-installed-formula>

e.g. the installed-on-request status of openssl@3 after:

$ brew install ruby
$ brew install openssl@3

And result would be different by changing order:

$ brew install openssl@3
$ brew install ruby

[cho-m]

Doesn't have to be in this PR, but one way of preserving original tab in case of migration via alias (not rename) is adding a fallback for keg

brew(main):001> Formula["pkg-config"].opt_prefix
=> #<Pathname:/usr/local/opt/pkgconf>
brew(main):002> Formula["pkg-config"].optlinked?
=> false
brew(main):003> Formula["pkg-config"].installed_kegs.find(&:optlinked?)
=> #<Keg:/usr/local/Cellar/pkg-config/0.29.2_3>
brew(main):004> Formula["pkg-config"].installed_kegs.find(&:optlinked?).tab.installed_as_dependency
=> true
brew(main):005> Formula["pkg-config"].installed_kegs.find(&:optlinked?).tab.installed_on_request
=> false

---

So maybe:

keg = if formula.optlinked?
  Keg.new(formula.opt_prefix.resolved_path)
else
  formula.installed_kegs.find(&:optlinked?)
end