[lldb][test][NFC] Add option to exclude third_party packages (#83191)

The goal here is to remove the third_party/Python/module tree, which
LLDB tests only use to `import pexpect`. This package is available on
`pip`, and I believe should not be hard to obtain.

However, in case it isn't easily available, deleting the tree right now
could cause disruption. This introduces a
`LLDB_TEST_USE_VENDOR_PACKAGES` cmake param that can be enabled, and the
tests will continue loading that tree. By default, it is enabled,
meaning there's really no change here. A followup change will disable it
by default once all known build bots are updated to include this
package. When disabled, an eager cmake check runs that makes sure
`pexpect` is available before waiting for the test to fail in an obscure
way.

Later, this option will go away, and when it does, we can delete the
tree too. Ideally this is not disruptive, and we can remove it in a week
or two.