Fetch all *CLASSPATH jars and dump their data into .pb files in the temp
directory shared between the target preparer and the testcases.
Test: atest ClasspathFetcherTest
Bug: 254647172
Change-Id: I008001975ba5febbec5fec20a908b1dcc02308f6
This will take over the responsibilities of StrictJavaPackagesTest's
setup code, and by fetching and processing all classpath data, as
defined in go/stricter-java-packages-test .
Test: atest ClasspathFetcherTest
Bug: 254647172
Change-Id: I93d2651994f7602d60ae91ca2401b83ae8c57baf