refactor(maven): Unify resource fetching utility functions

[zharinov]

Changes

Context

When fetching package info, Renovate fetches two POM-files:

• one for the latest version, in order to obtain actual homepage and sourceUrl
• another one for the release selected as the new versio, in order to obtain releaseTimestamp or reject this release if POM doesn't exist

In most cases, these files are same, so we may leverage caching to avoid extra HTTP calls. This PR contains the prior work before this objective could be achieved.

Documentation (please check one with an [x])

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How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
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