[ybai08]

Fixes #2158.

projectile-tasks-from-just only read the project's justfile, so recipes defined in just modules never showed up in projectile-run-task.

The provider now follows mod NAME and mod? NAME statements to the module source and offers its recipes qualified the way just names them: just:foo::build, running just foo::build.

• The module file is looked up where just looks for it: NAME.just, then mod.just, justfile, .justfile or Justfile in the NAME directory. An explicit path (mod foo 'tools/foo.just', single or double quoted) is used as given, and searched the same way when it names a directory.
• Paths resolve relative to the file declaring the module, so nested modules are followed (just:baz::qux::run).
• A module whose source is missing (the mod? case) is skipped, and so is a module that points back at a file already being read, so a loop can't recurse forever.
• The mod statement must end its line (an optional comment aside), so a recipe called mod that takes a parameter (mod target:) is still read as a recipe.
• As before, nothing shells out: the provider reads files only, so it stays cheap over TRAMP.

Specs added to test/projectile-tasks-test.el under projectile-tasks-from-just: modules (plain, mod? with a quoted path, an attribute line and a trailing comment, nested mod.just -> justfile), a missing module plus a self-referencing one, and a recipe called mod. The CHANGELOG.md entry and the discovered-tasks table in doc/modules/ROOT/pages/tasks.adoc are updated.

Checks (Emacs 31.1, emacs -Q --batch, buttercup/compat/consult from GNU ELPA/MELPA; Eldev itself was not used):

• test/projectile-tasks-test.el: on master the module spec fails (only just:default is returned); with the change 59 specs, 0 failed.
• All test/projectile-*-test.el: 1533 of 1553 specs ran, 0 failed (master: 1530 of 1550, 0 failed).
• projectile.el byte-compiles with byte-compile-error-on-warn; relint reports nothing; checkdoc reports nothing for the new functions.
• Not run: eldev test / eldev compile / eldev lint, and the module lookup was not checked against a real just binary (it follows the just manual's module section).

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