Rez depends -p will crash in some situations

[czerouni]

Describe the bug here.

Environment

• Rocky9
• Rez 3.2.1
• Rez python version 3.9.21

To Reproduce Running rez depends -p on a package that has private build requirements configured as a late-binding function will fail. The same package configured with private build requirements as a simple list will not fail.

This appears to be a bug in the packages_search.py file, which is assuming that all requirements are of type PackageRequest when some are actually of type str