RFR: 8258117: jar tool sets the time stamp of module-info.class entries to the current time

[Jaikiran Pai]

The commit here is a potential fix for the issue noted in https://bugs.openjdk.java.net/browse/JDK-8258117.

The change here repurposes an existing internal interface `ModuleInfoEntry` to keep track of the last modified timestamp of a `module-info.class` descriptor.

This commit uses the timestamp of the `module-info.class` on the filesystem to set the time on the `JarEntry`. There are a couple of cases to consider here:

1. When creating a jar  (using `--create`), we use the source `module-info.class` from the filesystem and then add extended info to it (attributes like packages, module version etc...). In such cases, this patch will use the lastmodified timestamp from the filesystem of `module-info.class` even though we might end up updating/extending/modifying (for example by adding a module version) its content while storing it as a `JarEntry`. 

2. When updating a jar (using `--update`), this patch will use the lastmodified timestamp of `module-info.class` either from the filesystem or from the source jar's entry (depending on whether a new `module-info.class` is being passed to the command). Here too, it's possible that we might end up changing/modifying/extending the `module-info.class` (for example, changing the module version to a new version) that gets written into the updated jar file, but this patch _won't_ use `System.currentTimeMillis()` even in such cases.

If we do have to track actual changes that might happen to `module-info.class` while extending its info (in `extendedInfoBytes()`) and then decide whether to use current system time as last modified time, then this will require a bigger change and also a discussion on what kind of extending of module-info.class content will require a change to the lastmodifiedtime of that entry.

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Commit messages:
 - 8258117: jar tool sets the time stamp of module-info.class entries to the current time

Changes: https://git.openjdk.java.net/jdk/pull/5486/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5486&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258117
  Stats: 317 lines in 2 files changed: 298 ins; 0 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5486.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5486/head:pull/5486

PR: https://git.openjdk.java.net/jdk/pull/5486