Support output format for list option

[cronik]

Direct cli user messages to standard error rather than standard out. Standard out
is reserved for primary output, which in cases like --list --output yaml would
be the yaml formatted plugin list. This separation of output is consistent with
general cli guidelines, https://clig.dev/#the-basics.

Fixes: #377
Fixes #284
Fixes #227

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[timja]

lgtm, can you check the test failure please?

[timja]

How does this fix #227?

[cronik]

With this changeset the following command can be used to generate a plugin file with the current set of effective plugins.

jenkins-plugin-cli --list --output yaml > plugins.yaml

not sure if there is more to #227 that I'm missing or not aware of.

[timja]

That issue is about generating a plugin list from an existing Jenkins home.

Currently the CLI doesn't know anything about the Jenkins home only the war file, so if I run e.g. this:

java -jar plugin-management-cli/target/jenkins-plugin-manager-2.12.7-SNAPSHOT.jar --list --output txt  -d plugins --war /Users/timja/code/jenkins/jenkins/war/target/jenkins.war

I get:

Installed plugins:
-none-

Bundled plugins:
-none-

All requested plugins:
-none-

Plugins that will be downloaded:
-none-


Done

[timja]

While this PR looks great I don't think it fixes any of the linked issues.

It does fix an issue where in edge cases something gets printed to stdout and breaks the file.

e.g. a recent issue that fixed the single case:
#438

[MarkEWaite]

@timja this pull request works in at least one of the use cases that I believe was previously missing from the tool. I have a directory that contains plugin files but don't have a plugins.txt file that describes those plugins and their precise versions. With this pull request, I can now run the tool and generate a plugins.txt file based on the contents of that plugins directory.

Example use case looks like this:

$ git clone -b lts-with-plugins https://github.com/MarkEWaite/docker-lfs.git
$ cd docker-lfs/ref
$ wget https://home.markwaite.net/~mwaite/jenkins-plugin-manager-2.12.7-pre.jar
$ java -jar jenkins-plugin-manager-2.12.7-pre.jar -d plugins --jenkins-version 2.332.3 --list > plugins.txt
$ cat plugins.txt

I thought that was the use case described by #227, but may have missed something in my understanding

[timja]

Oh cool! That worked, checking again

[timja]

I think this should be documented in the README, especially that you need to redirect stderr to dev null

Is it worth adding an option to suppress stderr except on error? quiet? Can file as a separate issue.

Sorting the result would be great too (also could be a separate pull request)