Rework walking logic to support pattern field

[qfox]

• Support pattern field in the main walk method (like file.parse should actually do it?)
  • Parse pattern to find ${entity} inside it
  • Choose Walker according to passed scheme field (can be flat and nested for now)
  • Read basic directory to generate some context
  • Read the rest with chosen Walker
  • Filter anything unneeded from walker with pattern
  • Send to stream (as it was done before)
• Walkers should care about ${entity} part in pattern only
  • Tech should not be parsed in walkers now, move it
  • Walkers should receive basic depth/scope and path to go deeper (resolves Ability to read just some part of level instead of full walk bem-sdk-archive/bem-walk#14, To have a way to read single (or several) blocks files bem-sdk-archive/bem-walk#37)
• Empty pattern means ${layer}/${entity}.${tech}

Feels like it's all the things we should do for pattern support. Not a huge deal for real.

More ideas on Russian: bem-sdk-archive/bem-walk#77
More details on Russian: https://gist.github.com/zxqfox/02bacc8c5d44b425f819228cdf0756d3

Closes #282

[belozer]

pattern: '${entity}/${tech}', scheme: 'nested'

I correctly understand that this code describes such a file structure?

common.blocks/block/index.js
common.blocks/block/__elem/index.js
common.blocks/block/__elem/_mod/index.js
common.blocks/block/__elem/_mod_val/index.js

[qfox]

Almost. This:

common.blocks/block/js
common.blocks/block/__elem/js
common.blocks/block/__elem/_mod/js
common.blocks/block/__elem/_mod_val/js

You need: pattern: '${entity}/index.${tech}', scheme: 'nested'

[belozer]

I probably brought a very abstract example for fs.

From real world

common.blocks/block/vanilla.js 
// -> { entity : { block : 'block' }, tech : 'vanilla.js' }

common.blocks/block/__elem/browser.js 
// -> { entity : { block : 'block' , elem : 'elem' }, tech : 'browser.js' }

common.blocks/block/__elem/_mod/bemtree.js
// -> { entity : { block : 'block' , elem : 'elem', mod : { name : 'mod', val : true }, tech : 'bemtree.js' }

common.blocks/block/__elem/_mod_val/bemhtml.js
// -> { entity : { block : 'block' , elem : 'elem', mod : { name : 'mod', val : 'val' }, tech : 'bemhtml.js' }

[qfox]

Then your pattern is the correct one. ;-)

But scheme should be corrected a little. Nested will match these files:

common.blocks/block/__elem/_mod/block__elem_mod/bemtree.js
common.blocks/block/__elem/_mod/block__elem_mod_val/bemhtml.js