Bundle rework

[rm-dr]

Re-opening PR, original (#1121) was deleted by branch cleanup.

Our current bundle scheme is causing quite a few bugs. This PR (paired with tectonic-typesetting/tectonic-texlive-bundles#14) resolves many of these by implementing proper search paths in bundles.

These changes are not be breaking. Zip and tar bundles may still be used, but they can no longer be created.

Changes in this PR:

• A new bundle format (see Huge rewrite, many fixes tectonic-texlive-bundles#14)
• File search paths (also documented in the bundle repo)
• Bundle autodetection from url (--web-bundle has been removed, and --bundle now handles both URLs and paths)
• Bundle caching rework (invisible to the user, but cleans up code)
• Cache reorganization
• Bundle-building code is now included in Tectonic as tectonic bundle create

Most changes in this PR are in the bundles crate. Changes in other files are usually minor api side-effects.

[codecov]

Codecov Report

Attention: Patch coverage is 22.42249% with 1076 lines in your changes missing coverage. Please review.

Project coverage is 46.20%. Comparing base (ac15f72) to head (a5e3416).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
...in/tectonic/v2cli/commands/bundle/select/picker.rs	0.00%	292 Missing ⚠️
crates/bundles/src/ttb.rs	0.00%	137 Missing ⚠️
crates/bundles/src/ttb_net.rs	0.00%	108 Missing ⚠️
...in/tectonic/v2cli/commands/bundle/pack/bundlev1.rs	0.00%	102 Missing ⚠️
src/bin/tectonic/v2cli/commands/bundle/actions.rs	0.00%	74 Missing ⚠️
crates/bundles/src/ttb_fs.rs	0.00%	62 Missing ⚠️
crates/bundles/src/cache.rs	69.34%	42 Missing ⚠️
crates/bundles/src/itar.rs	72.09%	36 Missing ⚠️
...tectonic/v2cli/commands/bundle/select/input/tar.rs	0.00%	35 Missing ⚠️
.../bin/tectonic/v2cli/commands/bundle/select/spec.rs	0.00%	29 Missing ⚠️
... and 16 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1218      +/-   ##
==========================================
- Coverage   46.95%   46.20%   -0.75%     
==========================================
  Files         177      188      +11     
  Lines       65197    66146     +949     
==========================================
- Hits        30613    30564      -49     
- Misses      34584    35582     +998     

Flag	Coverage Δ
	46.20% <22.42%> (-0.75%)	⬇️

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