Add support for creating new projects with existing files

We also recognize and handle the following existing entries in the project
root:

  .git      -- assume VCS is already initialized and is Git

  LICENSE   -- try to guess the license from contents

  README.md -- try to extract the summary line from contents

Overall, the idea is to streamline the workflow where one creates a project on
one of the hosting services (GitHub, GitLab) and then initializes it with
bdep-new.

Also, to this effect, specifying the project name is now optional and if
omitted the current working directory (or --output-dir, if specified) is
assumed to be the project name.

Here is an example of this workflow:

$ # create project with LICENSE and README.md on Git hosting service
$ git clone .../libhello.git
$ cd libhello
$ bdep new -t lib

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/new.testscript b/tests/new.testscript
index 8a6ff65..161fda1 100644
--- a/tests/new.testscript
+++ b/tests/new.testscript
@@ -230,6 +230,38 @@ status += -d prj
       EOE
   }
 
+  # Test create-from-existin functionality.
+  #
+  : exist
+  :
+  {
+    : basics
+    :
+    mkdir libfoo &!libfoo/;
+    git -C libfoo init -q;
+    cat <<EOI >=libfoo/README.md &!libfoo/README.md;
+    # libfoo
+
+    cool foo
+
+    Some more stuff.
+    EOI
+    cat <<EOI >=libfoo/LICENSE &!libfoo/LICENSE;
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+    TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+    ...
+    EOI
+    $* -t lib --output-dir libfoo 2>>/"EOE" &libfoo/***;
+      created new library project libfoo in $~/libfoo/
+      EOE
+    test -f libfoo/.gitignore;
+    sed -n -e 's/^summary: (.+)$/\1/p'      libfoo/manifest >'cool foo';
+    sed -n -e 's/^license: ([^ ]+).*$/\1/p' libfoo/manifest >'ASLv2'
+  }
+
   : pkg
   :
   {