Explicitly support absolute swDest paths

[jeffposnick]

R: @addyosmani @gauntface
CC: @freezy

At least one of the two issues reported in #1367 are due to webpack hanging when an absolute Windows path is used as a new asset filename. This PR should address that scenario by translating absolute paths into paths that are relative to the output directory.

[workbox-pr-bot]

PR-Bot Size Plugin

Changed File Sizes

File	Before	After	Change	GZipped
packages/workbox-build/build/index.js	2.52 KB	4.03 KB	+60%	1.57 KB	☠️
packages/workbox-webpack-plugin/build/generate-sw.js	7.38 KB	7.54 KB	+2%	2.48 KB
packages/workbox-webpack-plugin/build/inject-manifest.js	8.96 KB	9.11 KB	+2%	2.96 KB

New Files

No new files have been added.

All File Sizes

View Table

File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.51 KB	3.51 KB	0%	1.45 KB
packages/workbox-broadcast-cache-update/build/workbox-broadcast-cache-update.prod.js	1.12 KB	1.12 KB	0%	584 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/index.js	2.52 KB	4.03 KB	+60%	1.57 KB	☠️
packages/workbox-cache-expiration/build/workbox-cache-expiration.prod.js	3.40 KB	3.40 KB	0%	1.25 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	587 B	587 B	0%	345 B
packages/workbox-cli/build/app.js	6.50 KB	6.50 KB	0%	2.16 KB
packages/workbox-cli/build/bin.js	2.32 KB	2.32 KB	0%	1.03 KB
packages/workbox-core/build/workbox-core.prod.js	6.47 KB	6.47 KB	0%	2.59 KB
packages/workbox-google-analytics/build/workbox-google-analytics.prod.js	2.13 KB	2.13 KB	0%	1.05 KB
packages/workbox-precaching/build/workbox-precaching.prod.js	5.66 KB	5.66 KB	0%	2.17 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.64 KB	1.64 KB	0%	802 B
packages/workbox-routing/build/workbox-routing.prod.js	2.77 KB	2.77 KB	0%	1.25 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	3.37 KB	3.37 KB	0%	1.03 KB
packages/workbox-sw/build/workbox-sw.js	1.45 KB	1.45 KB	0%	788 B
packages/workbox-webpack-plugin/build/generate-sw.js	7.38 KB	7.54 KB	+2%	2.48 KB
packages/workbox-webpack-plugin/build/index.js	742 B	742 B	0%	470 B
packages/workbox-webpack-plugin/build/inject-manifest.js	8.96 KB	9.11 KB	+2%	2.96 KB

Workbox Aggregate Size Plugin

☠️ WARNING ☠️

We are using 158% of our max size budget.

Total Size: 23.15KB
Percentage of Size Used: 158%

Gzipped: 9.21KB

[coveralls]

Coverage increased (+0.03%) to 88.39% when pulling 12127bc on ensure-swdest-relative into ed2a847 on master.