Don't keep AST in memory, lazy load content on demand

[felixfbecker]

Extension of #60

Now only a FQN => PhpDocument map is kept at the project level.
Upon a definition request the content/AST of the document is lazily loaded from disk.

Indexing is still slow but RAM usage is now as low as <100MB even for magento (for some reason though VS Code crashed after ~20,000 files).

I moved all LSP-related logic like findSymbols out of Project and PhpDocument into the method handlers. Project and PhpDocument in turn provide methods like getDefinition or getDefinitionDocument.

workspace/symbol now also searches over the FQN, which is a nice side-effect.
The downside though is that workspace/symbol now needs to reparse every document matching the query. So in the case that your query is not very specific, it will take forever in large projects and bring your RAM usage up to the sky.

Test with Symfony (7100 files):

All PHP files parsed in 259 seconds. 52 MiB allocated.

[codecov-io]

Current coverage is 90.33% (diff: 93.10%)

Merging #61 into master will increase coverage by 0.41%

@@             master        #61   diff @@
==========================================
  Files            23         24     +1   
  Lines           496        507    +11   
  Methods          70         73     +3   
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            446        458    +12   
+ Misses           50         49     -1   
  Partials          0          0          

Diff Coverage	File Path
•••••••• 89%	src/PhpDocument.php
•••••••••• 100%	src/Server/TextDocument.php
•••••••••• 100%	src/Server/Workspace.php
•••••••••• 100%	src/Project.php
•••••••••• 100%	src/LanguageServer.php
•••••••••• 100%	src/Protocol/SymbolInformation.php

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[mniewrzal]

With this PR I have reasonable indexing times:
magento

[Info  - 9:57:14 PM] Parsing file 24273/24273: dev/tests/functional/tests/app/Magento/Email/Test/Constraint/AssertEmailTemplateSuccessSaveMessage.php.
[Info  - 9:57:14 PM] All PHP files parsed in 119 seconds. 575 MiB allocated.

symfony

[Info  - 9:58:36 PM] Parsing file 7099/7100: vendor/zendframework/zend-code/test/Generic/Prototype/PrototypeClassFactoryTest.php.
[Info  - 9:58:36 PM] Parsing file 7100/7100: .github/build-packages.php.
[Info  - 9:58:36 PM] All PHP files parsed in 27 seconds. 50 MiB allocated.

To be clear I'm testing with XDebug disabled.