fix: support build cache

[pi0]

Fix issues with nuxt build cache for static target.

Related issues: #367, #352, #353

This PR mainly fixes #353

When using nuxt generate and cache hit happens, we skip nuxt build step and directly generate. However as build:* hooks won't be called anymore (even tough buildModules are called!)

Attempts to fix

0. 491f440 ensures sw.js is properly emitted (temporary fix)
1. Run pwa modules both if _build flag exists (nuxt dev, nuxt build and first optional stage of nuxt generate) or target is static. Downside is we call modules twice when cache miss occurs but seems only way
2. Directly emit files and generate template as even tough buildModules are called for generate's second stage builder is not loaded so webpack hook to emit assets and addTemplate does not work

Known issues

icon

Nuxt does not checks static/ dir (static/icon.png) for build cache. This means if icon is updated, module wasn't being called. By always running module this should be fixed

manifest

Relying on webpack hook means we can't generate updated manifest (in case icon hash changes)

meta

If meta depending on process.env, we won't regenerate it as of addTemplate builder dependency

workbox

sw.js file needs to be restored. Instead of 491f440 workaround, we can directly generate template without depending on nuxt builder

[codecov]

Codecov Report

Merging #371 into master will increase coverage by 0.12%.
The diff coverage is 95.00%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   84.75%   84.87%   +0.12%     
==========================================
  Files          11       12       +1     
  Lines         400      390      -10     
  Branches      119      119              
==========================================
- Hits          339      331       -8     
+ Misses         55       54       -1     
+ Partials        6        5       -1     

Impacted Files	Coverage Δ
lib/manifest/module.js	90.90% <85.71%> (-2.20%)	⬇️
lib/module.js	83.33% <85.71%> (-4.91%)	⬇️
lib/workbox/options.js	90.19% <87.50%> (ø)
lib/icon/module.js	92.40% <95.45%> (-0.78%)	⬇️
lib/meta/module.js	78.35% <100.00%> (-0.60%)	⬇️
lib/meta/module.runtime.js	100.00% <100.00%> (ø)
lib/utils/index.js	92.30% <100.00%> (+3.84%)	⬆️
lib/workbox/module.js	100.00% <100.00%> (+4.54%)	⬆️
lib/workbox/utils.js	94.73% <100.00%> (+1.40%)	⬆️

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[pi0]

I tried to locally test with different situations, Still changes by this PR might introduce some regressions. In that case please make an issue :)