[3.3] Fixes for .bolt.yml / .bolt.php custom path handling

[GwendolenLynch]

Fixes #6797

OK, so this was a bit of fun to debug with a brain that is only coming back into business mode … not to mention keeping backward and forward compatibility working. 🤣

This PR addresses the first problem I hit with BootstrapYamlUpdater updating custom path handling:

• Add forward-compatible path resolution for var/ (it's in active use in the wild on 3.2+ sites)
• Handle FC path updating for use of var/
• Don't assume that a path that defaults to being under app/ is actually under app/
• Adds a note on debugging Composer for ageing princesses with memory recall problems 😜

Bolt 3.2.x use/test case, taken from a production site:

paths:
    var: var
    cache: var/cache
    config: app/config
    database: app/database
    web: web
    themebase: web/theme
    files: web/files
    view: web/bolt-web/view

[bobdenotter]

Did a bunch of monkey-testing, works fine for me. 🐵

[GwendolenLynch]

Reviewed and approved by @bobdenotter and in-principle agreement at last night's meeting from @CarsonF … so sending in for today's march toward 3.3.