Restore legacy variables

[mamoll]

Rebased version of #1185.
Manually resolved a merge conflict.
See #1185 for details

[saahu27]

@Ryanf55 @mamoll I can confirm, The changes do not break the existing build. But as these changes are made to support legacy users, i am trying to understand how to verify these changes for legacy users. Please advice. Thank you.

[saahu27]

@mamoll @Ryanf55 @MarqRazz Any build system compiling a package looks for a dependency in the packages config.cmake.in. Because of the changes made to OMPL CMakeLists to modernize the package management, the look up in other packages should be like ompl::ompl. Since, older versions use ompl to look up, this broke the legacy systems.

The above commit enables both the look ups to point to the right libraries. this way both the legacy systems and the new modern approach both work.

I have tested the build and plan, both in humble and also jazzy versions of moveit. I believe this solves the issue we are trying to solve.

Could anyone help verify this changes?

[MarqRazz]

Tested and it works with both MoveIt2 methods, main and my PR 🎉.

Do we want to add any deprecation information here about the old method? Any notes you want to add to the ReleaseNotes?

[saahu27]

@MarqRazz Thank you for verifying the changes.

This pull request gives access to deprecated methods. (find_package(ompl)).
I am working on the release, which include updates to website, release notes. i will make a mention of this in the release notes.