Support read-only buildpack cache

If a repository file cannot be downloaded, look for the file in
the read-only buildpack cache and use that file if it exists.

This code is localised in a look_aside method for ease of stripping
out once a read-write buildpack cache is implemented.

[#54290822]
Merge 54290822-no-external-connection to master

[#54290822]
Test use of stashed index.yml

[#54290822]
Merge 54290822-no-external-connection to master

[#54290822]
Merge branch 'master' of github.com:cloudfoundry/java-buildpack