refactor: use walkdir instead of globby

[JustinBeckwith]

Should have a ~120ms reduction in module load time.

[ofrobots]

LGTM. Hopefully a follow-on can return a Promise of files rather than doing sync work. Who uses this feature anyway? Is the caching actually worth it?

[JustinBeckwith]

I have no idea my friend. I don't have a good handle on what this library does vs gax. I will however follow up with an async version after this all propagates without incident. It's very core, so I'm scared to change a ton at once.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9a23d42). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #109   +/-   ##
=========================================
  Coverage          ?   91.54%           
=========================================
  Files             ?        6           
  Lines             ?      201           
  Branches          ?        6           
=========================================
  Hits              ?      184           
  Misses            ?       17           
  Partials          ?        0

Impacted Files	Coverage Δ
src/load.ts	100% <100%> (ø)

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