fix: tighten up rollup bundles

[tivac]

When working on #449 I got annoyed at how many times I was having to check common.css in cases where it wasn't actually necessary.

So now I've done something about it. modular-css-rollup will only create a common.css if there's more than one CSS bundle being generated which is how it should've been all along.

[codecov]

Codecov Report

Merging #450 into master will decrease coverage by 0.12%.
The diff coverage is 93.75%.

@@           Coverage Diff            @@
##           master   #450      +/-   ##
========================================
- Coverage   99.12%    99%   -0.13%     
========================================
  Files          31     31              
  Lines         798    800       +2     
  Branches      123    124       +1     
========================================
+ Hits          791    792       +1     
- Misses          7      8       +1

Impacted Files	Coverage Δ
packages/rollup/rollup.js	98.73% <93.75%> (-1.27%)	⬇️

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