Make the grep dependency honest; drop unused requires

[bbatsov]

This was #2 on the list ("defer the eager requires for startup"), but the investigation changed the conclusion, so the scope is smaller and honestly framed.

Finding: the eager requires can't be deferred for a startup win. helm-projectile builds Helm sources at load (top-level defclass/helm-make-source), so Helm is a hard load-time dependency, and Helm's own module graph transitively loads grep, helm-locate and helm-global-bindings regardless. I verified (featurep 'grep) etc. are all t right after (require 'helm-projectile) even with the explicit requires removed. Deferring would need every source definition restructured to be lazy - too big/risky for the payoff.

What this does instead (dependency hygiene, no perf claim):

• require 'grep from helm-projectile--ignored-files / -directories, the two helpers that actually read grep-find-ignored-files/-directories, so they're self-sufficient instead of relying on a transitive load that could change. (Also finally retires the years-old TODO: defer this require.)
• Drop the helm-locate and helm-global-bindings requires - nothing here uses a symbol from either.

Load cost is unchanged; the dependencies are just honest now. Compile (warnings-as-errors), checkdoc and 66 specs all pass.

• The commits are consistent with our contribution guidelines
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog
• You've updated the readme (no user-facing change)

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.33%. Comparing base (c4e4ea8) to head (c568ae1).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   46.14%   46.33%   +0.19%     
==========================================
  Files           1        1              
  Lines         557      559       +2     
==========================================
+ Hits          257      259       +2     
  Misses        300      300              

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