load xml as string from local file, fixes #3942

[alcohol]

If the entity loader is disabled on a system, loading files, even from the local file system, is considered as external to the running php process, and thus not allowed by the libxml extension. Reading the file contents and loading the xml as a string is a valid alternative however.

Fixes #3942

[stof]

👍

[alcohol]

Modified the unit tests to simulate the scenario in which previously the PearPackageExtractor would fail. Tests failed with the fix reverted, pass with the fix.

/srv/git/github/composer (load-xml-as-string) $ vendor/bin/phpunit tests/Composer/Test/Downloader/PearPackageExtractorTest.php
PHPUnit 4.6.6 by Sebastian Bergmann and contributors.

Configuration read from /srv/git/github/composer/phpunit.xml.dist

EEE.

Time: 128 ms, Memory: 5.00Mb

There were 3 errors:

1) Composer\Test\Downloader\PearPackageExtractorTest::testShouldExtractPackage_1_0
simplexml_load_file(): I/O warning : failed to load external entity "/srv/git/github/composer/tests/Composer/Test/Downloader/Fixtures/Package_v1.0/package.xml"

/srv/git/github/composer/src/Composer/Downloader/PearPackageExtractor.php:140
/srv/git/github/composer/tests/Composer/Test/Downloader/PearPackageExtractorTest.php:27

2) Composer\Test\Downloader\PearPackageExtractorTest::testShouldExtractPackage_2_0
simplexml_load_file(): I/O warning : failed to load external entity "/srv/git/github/composer/tests/Composer/Test/Downloader/Fixtures/Package_v2.0/package.xml"

/srv/git/github/composer/src/Composer/Downloader/PearPackageExtractor.php:140
/srv/git/github/composer/tests/Composer/Test/Downloader/PearPackageExtractorTest.php:62

3) Composer\Test\Downloader\PearPackageExtractorTest::testShouldExtractPackage_2_1
simplexml_load_file(): I/O warning : failed to load external entity "/srv/git/github/composer/tests/Composer/Test/Downloader/Fixtures/Package_v2.1/package.xml"

/srv/git/github/composer/src/Composer/Downloader/PearPackageExtractor.php:140
/srv/git/github/composer/tests/Composer/Test/Downloader/PearPackageExtractorTest.php:85

FAILURES!
Tests: 4, Assertions: 1, Errors: 3.