Don't lazily retrieve gem specs for completion

[st0012]

There are a few downsides of the current approach:

1. Because gem specs are lazily retrieved, this computation happens in every irb completion test case, which is not necessary. (In tests we don't cache the result of retrieve_files_to_require_from_load_path)
2. Gem::Specification.latest_specs is sensible to the content of LOAD_PATH. And when combined with 1, tests fail "randomly" if they try to mutate LOAD_PATH, even though the test subject it's something else.

So by pre-computing and storing the gem paths in a constant, it guarantees that the computation only happens once and it doesn't get affected by test cases.

One argument could be made against the change is that, it'll store unnecessary data for users that disable autocompletion. But the counter-arguments are:

1. Since autocompletion is enabled by default, this should not be the case for most users.
2. For users with autocompletion enabled, IRB already caches the result of retrieve_files_to_require_from_load_path in memory, which should have a similar size of GEM_SPECS. And we currently haven't received any report about problems caused by such memory consumption.

Testing With Ruby

This script:

$ make test-all -j8 TESTS=../test/irb/ TESTOPTS="--no-retry --seed=31291"

can reproduce the related test failures with Ruby:

# Running tests:


  1) Error:
TestIRB::TestCompletion::TestRequireComepletion#test_complete_require_with_string_convertable_in_load_path:
Test::Unit::ProxyError: no implicit conversion of Object into String
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems.rb:364:in `paths'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems.rb:408:in `path'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems/specification.rb:965:in `dirs'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems/specification.rb:856:in `stubs_for_pattern'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems/specification.rb:819:in `stubs'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems/specification.rb:1118:in `latest_specs'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/irb/completion.rb:76:in `retrieve_gem_and_system_load_path'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/irb/completion.rb:99:in `retrieve_files_to_require_from_load_path'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/irb/completion.rb:139:in `block in <module:InputCompletor>'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/irb/completion.rb:157:in `block in <module:InputCompletor>'
    /Users/hung-wulo/src/github.com/Shopify/ruby/test/irb/test_completion.rb:114:in `test_complete_require_with_string_convertable_in_load_path'

  2) Error:
TestIRB::TestCompletion::TestRequireComepletion#test_complete_require_with_malformed_object_in_load_path:
Test::Unit::ProxyError: no implicit conversion of Object into String
    <internal:/Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems/core_ext/kernel_require.rb>:85:in `require'
    <internal:/Users/hung-wulo/src/github.com/Shopify/ruby/lib/rubygems/core_ext/kernel_require.rb>:85:in `require'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/pp.rb:76:in `width_for'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/pp.rb:91:in `pp'
    /Users/hung-wulo/src/github.com/Shopify/ruby/lib/pp.rb:638:in `pretty_inspect'

But with this patch both of them should pass.