Fix work without pkg-config on mac

[gavv]

This PR fixes OpenSSL search on macos when pkg-config is not installed, which happens randomly on some (but not all) github action workers.

Without this PR, OpenSSL can be used without pkg-config on linux, but not on mac, because on mac OpenSSL is inside brew prefix.

Changes:

• rename pkgconfig.py to systemdeps.py
• move FindBrewPackage to systemdeps.py
• replace AddPkgConfigLibs with AddManualDependency, which can handle not only libs, but also custom prefix
• use AddManualDependency with ossl_prefix if OpenSSL was not found using pkg-config