cli: list installed plugins and entry points

[oliver-sanders]

List installed plugins in cylc version --long.

Addresses cylc/cylc-admin#76.

$ cylc version --long
8.0b0 (/Users/oliver/miniforge/envs/cylc8)

Plugins:
  cylc-rose       0.1.0   /Users/oliver/cylc-rose
  cylc-uiserver   0.3.0   /Users/oliver/cylc-uiserver

Entry Points:
  cylc.command:
    hub = cylc.uiserver.scripts.hub:main
    uiserver = cylc.uiserver.scripts.uis:main
  cylc.post_install:
    rose_opts = cylc.rose.entry_points:post_install
  cylc.pre_configure:
    rose = cylc.rose.entry_points:pre_configure

Plugins are detected providing that they provide at least one cylc.* entry point and are not contained within the cylc.flow distribution.

Context:

We have now settled on the following points which we weren't clear on near the start of the project:

• The cylc-meta version will match the cylc-flow version.
• We should expect cylc-flow to be installed in all places where cylc-meta is installed.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Appropriate tests are included (tricky but possible to test)
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.
• No dependency changes.

[oliver-sanders]

Can't blame shellcheck for the Bash unsafe-by-defaut variable templating model!

[hjoliver]

Nice.

For testing (follow-up) I guess we could just install a small plugin into the test environment, and check the long form output?

[oliver-sanders]

Might need to mock the two entry point discovery methods from pkg_resources to filter out any other plugins users may have installed. Will have a think.