o/snapstate: reorder tasks such that snap and component downloads now happen consecutively

[andrewphelpsj]

This change makes it so when installing (or refreshing) a snap and its components, we do all the downloads back-to-back. This will help make the code changes needed for remodeling simpler.

Example of the tasks created when installing a snap and two components from the store:

task prerequisites (1), Ensure prerequisites for "test-snap" are available
  waiting on:
task download-snap (2), Download snap "test-snap" (11) from channel "channel-for-components"
  waiting on:
  - prerequisites (1)
task validate-snap (13), Fetch and check assertions for snap "test-snap" (11)
  waiting on:
  - download-snap (2)
task download-component (3), Download component "kernel-modules-component" (1)
  waiting on:
  - validate-snap (13)
task validate-component (4), Fetch and check assertions for component "kernel-modules-component" (1)
  waiting on:
  - download-component (3)
task download-component (8), Download component "standard-component" (2)
  waiting on:
  - validate-component (4)
task validate-component (9), Fetch and check assertions for component "standard-component" (2)
  waiting on:
  - download-component (8)
task mount-snap (14), Mount snap "test-snap" (11)
  waiting on:
  - validate-component (9)
task mount-component (5), Mount component "test-snap+kernel-modules-component" (1)
  waiting on:
  - validate-component (4)
  - mount-snap (14)
task mount-component (10), Mount component "test-snap+standard-component" (2)
  waiting on:
  - validate-component (9)
  - mount-component (5)
task copy-snap-data (15), Copy snap "test-snap" data
  waiting on:
  - mount-component (10)
task setup-profiles (16), Setup snap "test-snap" (11) security profiles
  waiting on:
  - copy-snap-data (15)
task link-snap (17), Make snap "test-snap" (11) available to the system
  waiting on:
  - setup-profiles (16)
task link-component (6), Make component "test-snap+kernel-modules-component" (1) available to the system
  waiting on:
  - mount-component (5)
  - link-snap (17)
task link-component (11), Make component "test-snap+standard-component" (2) available to the system
  waiting on:
  - mount-component (10)
  - link-component (6)
task auto-connect (18), Automatically connect eligible plugs and slots of snap "test-snap"
  waiting on:
  - link-component (11)
task set-auto-aliases (19), Set automatic aliases for snap "test-snap"
  waiting on:
  - auto-connect (18)
task setup-aliases (20), Setup snap "test-snap" aliases
  waiting on:
  - set-auto-aliases (19)
task run-hook (21), Run install hook of "test-snap" snap if present
  waiting on:
  - setup-aliases (20)
task run-hook (7), Run install hook of "test-snap+kernel-modules-component" component if present
  waiting on:
  - link-component (6)
  - run-hook (21)
task run-hook (12), Run install hook of "test-snap+standard-component" component if present
  waiting on:
  - link-component (11)
  - run-hook (7)
task prepare-kernel-modules-components (22), Prepare kernel-modules components for "test-snap" (11)
  waiting on:
  - run-hook (12)
task run-hook (23), Run default-configure hook of "test-snap" snap if present
  waiting on:
  - prepare-kernel-modules-components (22)
task start-snap-services (24), Start snap "test-snap" (11) services
  waiting on:
  - run-hook (23)
task run-hook (25), Run configure hook of "test-snap" snap if present
  waiting on:
  - prerequisites (1)
  - download-snap (2)
  - validate-snap (13)
  - download-component (3)
  - validate-component (4)
  - download-component (8)
  - validate-component (9)
  - mount-snap (14)
  - mount-component (5)
  - mount-component (10)
  - copy-snap-data (15)
  - setup-profiles (16)
  - link-snap (17)
  - link-component (6)
  - link-component (11)
  - auto-connect (18)
  - set-auto-aliases (19)
  - setup-aliases (20)
  - run-hook (21)
  - run-hook (7)
  - run-hook (12)
  - prepare-kernel-modules-components (22)
  - run-hook (23)
  - start-snap-services (24)
task run-hook (26), Run health check of "test-snap" snap
  waiting on:
  - prerequisites (1)
  - download-snap (2)
  - validate-snap (13)
  - download-component (3)
  - validate-component (4)
  - download-component (8)
  - validate-component (9)
  - mount-snap (14)
  - mount-component (5)
  - mount-component (10)
  - copy-snap-data (15)
  - setup-profiles (16)
  - link-snap (17)
  - link-component (6)
  - link-component (11)
  - auto-connect (18)
  - set-auto-aliases (19)
  - setup-aliases (20)
  - run-hook (21)
  - run-hook (7)
  - run-hook (12)
  - prepare-kernel-modules-components (22)
  - run-hook (23)
  - start-snap-services (24)
  - run-hook (25)

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (96ea7b0) to head (c262d11).
Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
overlord/snapstate/snapstate.go	85.71%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14697      +/-   ##
==========================================
+ Coverage   78.95%   78.97%   +0.01%     
==========================================
  Files        1084     1085       +1     
  Lines      146638   147155     +517     
==========================================
+ Hits       115773   116210     +437     
- Misses      23667    23720      +53     
- Partials     7198     7225      +27     

Flag	Coverage Δ
unittests	78.97% <87.50%> (+0.01%)	⬆️

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