Add support for --prefer-source Composer argument

[kinglozzer]

Note: this didn’t actually solve the problem below for me (as --prefer-source failed and fell back to dist, I’m looking into why - I think it may be a caching thing), but I thought this might be worth proposing anyway.

Problem: I have two packages, lets say Package A and Package B. Package A has a tests/TestCase.php, Package B depends on Package A and Package B’s test classes extend Package A’s TestCase:

// ./package-a/tests
class TestCase extends SapphireTest {
}

// ./package-b/tests
class SomeTest extends TestCase {
}

N.b: I’m not sure if this is a strange pattern? Doesn’t seem unreasonable to me...

If Package A has a .gitattributes that excludes the tests directory from exports, then composer install --prefer-dist will not include that directory either, so Package B’s tests will fail as TestCase doesn’t exist.

This PR adds the option to use --prefer-source for Composer installs, which clones the git repo so will include everything (it defaults to the current behaviour of --prefer-dist).

[helpfulrobot]

@kinglozzer, thanks for your PR! By analyzing the blame information on this pull request, I identified @sminnee and @chillu to be potential reviewers

[codecov-io]

Current coverage is 100%

Merging #35 into master will not change coverage

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[dhensby]

This patch seems completely reasonable though i suspect you problem is the prefer-stable setting :/