main: Also support CLI extensions in /usr/libexec/libostree/ext

In fixing https://github.com/coreos/rpm-ostree/pull/3323
I felt that it was a bit ugly we're installing `/usr/bin/ostree-container`.

It's kind of an implementation detail.  We want users to use
`ostree container`.

Let's support values outside of $PATH too.

For example, this also ensures that TAB completion for `ost` expands
to `ostree ` with a space.

src/libostree/ostree-1.pc.in

@@ -3,6 +3,7 @@ exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
 features=@OSTREE_FEATURES@
+cliextdir=@pkglibexecdir@/ext
 
 Name: OSTree
 Description: Git for operating system binaries